c#

.net

unicode

performance

algorithm

I've just had to write a string reverse function in C# 2.0 (i.e. LINQ not available) and came up with this:

public string Reverse(string text)
{
    char[] cArray = text.ToCharArray();
    string reverse = String.Empty;
    for (int i = cArray.Length - 1; i > -1; i--)
    {
        reverse += cArray[i];
    }
    return reverse;
}

Personally I'm not crazy about the function and am convinced that there's a better way to do it. Is there?

Solution 1

public static string Reverse( string s )
{
    char[] charArray = s.ToCharArray();
    Array.Reverse( charArray );
    return new string( charArray );
}

Solution 2

Here a solution that properly reverses the string "Les Mise\u0301rables" as "selbare\u0301siM seL". This should render just like selbarésiM seL, not selbaŕesiM seL (note the position of the accent), as would the result of most implementations based on code units (Array.Reverse, etc) or even code points (reversing with special care for surrogate pairs).

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;

public static class Test
{
    private static IEnumerable<string> GraphemeClusters(this string s) {
        var enumerator = StringInfo.GetTextElementEnumerator(s);
        while(enumerator.MoveNext()) {
            yield return (string)enumerator.Current;
        }
    }
    private static string ReverseGraphemeClusters(this string s) {
        return string.Join("", s.GraphemeClusters().Reverse().ToArray());
    }

    public static void Main()
    {
        var s = "Les Mise\u0301rables";
        var r = s.ReverseGraphemeClusters();
        Console.WriteLine(r);
    }
}

(And live running example here: https://ideone.com/DqAeMJ)

It simply uses the .NET API for grapheme cluster iteration, which has been there since ever, but a bit "hidden" from view, it seems.

Solution 3

This is turning out to be a surprisingly tricky question.

I would recommend using Array.Reverse for most cases as it is coded natively and it is very simple to maintain and understand.

It seems to outperform StringBuilder in all the cases I tested.

public string Reverse(string text)
{
   if (text == null) return null;

   // this was posted by petebob as well 
   char[] array = text.ToCharArray();
   Array.Reverse(array);
   return new String(array);
}

There is a second approach that can be faster for certain string lengths which uses Xor.

    public static string ReverseXor(string s)
    {
        if (s == null) return null;
        char[] charArray = s.ToCharArray();
        int len = s.Length - 1;

        for (int i = 0; i < len; i++, len--)
        {
            charArray[i] ^= charArray[len];
            charArray[len] ^= charArray[i];
            charArray[i] ^= charArray[len];
        }

        return new string(charArray);
    }

Note If you want to support the full Unicode UTF16 charset read this. And use the implementation there instead. It can be further optimized by using one of the above algorithms and running through the string to clean it up after the chars are reversed.

Here is a performance comparison between the StringBuilder, Array.Reverse and Xor method.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;

namespace ConsoleApplication4
{
    class Program
    {
        delegate string StringDelegate(string s);

        static void Benchmark(string description, StringDelegate d, int times, string text)
        {
            Stopwatch sw = new Stopwatch();
            sw.Start();
            for (int j = 0; j < times; j++)
            {
                d(text);
            }
            sw.Stop();
            Console.WriteLine("{0} Ticks {1} : called {2} times.", sw.ElapsedTicks, description, times);
        }

        public static string ReverseXor(string s)
        {
            char[] charArray = s.ToCharArray();
            int len = s.Length - 1;

            for (int i = 0; i < len; i++, len--)
            {
                charArray[i] ^= charArray[len];
                charArray[len] ^= charArray[i];
                charArray[i] ^= charArray[len];
            }

            return new string(charArray);
        }

        public static string ReverseSB(string text)
        {
            StringBuilder builder = new StringBuilder(text.Length);
            for (int i = text.Length - 1; i >= 0; i--)
            {
                builder.Append(text[i]);
            }
            return builder.ToString();
        }

        public static string ReverseArray(string text)
        {
            char[] array = text.ToCharArray();
            Array.Reverse(array);
            return (new string(array));
        }

        public static string StringOfLength(int length)
        {
            Random random = new Random();
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < length; i++)
            {
                sb.Append(Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65))));
            }
            return sb.ToString();
        }

        static void Main(string[] args)
        {

            int[] lengths = new int[] {1,10,15,25,50,75,100,1000,100000};

            foreach (int l in lengths)
            {
                int iterations = 10000;
                string text = StringOfLength(l);
                Benchmark(String.Format("String Builder (Length: {0})", l), ReverseSB, iterations, text);
                Benchmark(String.Format("Array.Reverse (Length: {0})", l), ReverseArray, iterations, text);
                Benchmark(String.Format("Xor (Length: {0})", l), ReverseXor, iterations, text);

                Console.WriteLine();    
            }

            Console.Read();
        }
    }
}

Here are the results:

26251 Ticks String Builder (Length: 1) : called 10000 times.
33373 Ticks Array.Reverse (Length: 1) : called 10000 times.
20162 Ticks Xor (Length: 1) : called 10000 times.

51321 Ticks String Builder (Length: 10) : called 10000 times.
37105 Ticks Array.Reverse (Length: 10) : called 10000 times.
23974 Ticks Xor (Length: 10) : called 10000 times.

66570 Ticks String Builder (Length: 15) : called 10000 times.
26027 Ticks Array.Reverse (Length: 15) : called 10000 times.
24017 Ticks Xor (Length: 15) : called 10000 times.

101609 Ticks String Builder (Length: 25) : called 10000 times.
28472 Ticks Array.Reverse (Length: 25) : called 10000 times.
35355 Ticks Xor (Length: 25) : called 10000 times.

161601 Ticks String Builder (Length: 50) : called 10000 times.
35839 Ticks Array.Reverse (Length: 50) : called 10000 times.
51185 Ticks Xor (Length: 50) : called 10000 times.

230898 Ticks String Builder (Length: 75) : called 10000 times.
40628 Ticks Array.Reverse (Length: 75) : called 10000 times.
78906 Ticks Xor (Length: 75) : called 10000 times.

312017 Ticks String Builder (Length: 100) : called 10000 times.
52225 Ticks Array.Reverse (Length: 100) : called 10000 times.
110195 Ticks Xor (Length: 100) : called 10000 times.

2970691 Ticks String Builder (Length: 1000) : called 10000 times.
292094 Ticks Array.Reverse (Length: 1000) : called 10000 times.
846585 Ticks Xor (Length: 1000) : called 10000 times.

305564115 Ticks String Builder (Length: 100000) : called 10000 times.
74884495 Ticks Array.Reverse (Length: 100000) : called 10000 times.
125409674 Ticks Xor (Length: 100000) : called 10000 times.

It seems that Xor can be faster for short strings.

Solution 4

If you can use LINQ (.NET Framework 3.5+) than following one liner will give you short code. Don't forget to add using System.Linq; to have access to Enumerable.Reverse:

public string ReverseString(string srtVarable)
{
    return new string(srtVarable.Reverse().ToArray());
}

Notes:

  • not the fastest version - according to Martin Niederl 5.7 times slower than the fastest choice here.
  • this code as many other options completely ignores all sorts of multi-character combinations, so limit usage to homework assignments and strings which do not contain such characters. See another answer in this question for implementation that correctly handles such combinations.

Solution 5

If the string contains Unicode data (strictly speaking, non-BMP characters) the other methods that have been posted will corrupt it, because you cannot swap the order of high and low surrogate code units when reversing the string. (More information about this can be found on my blog.)

The following code sample will correctly reverse a string that contains non-BMP characters, e.g., "\U00010380\U00010381" (Ugaritic Letter Alpa, Ugaritic Letter Beta).

public static string Reverse(this string input)
{
    if (input == null)
        throw new ArgumentNullException("input");

    // allocate a buffer to hold the output
    char[] output = new char[input.Length];
    for (int outputIndex = 0, inputIndex = input.Length - 1; outputIndex < input.Length; outputIndex++, inputIndex--)
    {
        // check for surrogate pair
        if (input[inputIndex] >= 0xDC00 && input[inputIndex] <= 0xDFFF &&
            inputIndex > 0 && input[inputIndex - 1] >= 0xD800 && input[inputIndex - 1] <= 0xDBFF)
        {
            // preserve the order of the surrogate pair code units
            output[outputIndex + 1] = input[inputIndex];
            output[outputIndex] = input[inputIndex - 1];
            outputIndex++;
            inputIndex--;
        }
        else
        {
            output[outputIndex] = input[inputIndex];
        }
    }

    return new string(output);
}

Solution 6

Ok, in the interest of "don't repeat yourself," I offer the following solution:

public string Reverse(string text)
{
   return Microsoft.VisualBasic.Strings.StrReverse(text);
}

My understanding is that this implementation, available by default in VB.NET, properly handles Unicode characters.

Solution 7

Have a look at the wikipedia entry here. They implement the String.Reverse extension method. This allows you to write code like this:

string s = "olleh";
s.Reverse();

They also use the ToCharArray/Reverse combination that other answers to this question suggest. The source code looks like this:

public static string Reverse(this string input)
{
    char[] chars = input.ToCharArray();
    Array.Reverse(chars);
    return new String(chars);
}

Solution 8

Greg Beech posted an unsafe option that is indeed as fast as it gets (it's an in-place reversal); but, as he indicated in his answer, it's a completely disastrous idea.

That said, I'm surprised there is so much of a consensus that Array.Reverse is the fastest method. There's still an unsafe approach that returns a reversed copy of a string (no in-place reversal shenanigans) significantly faster than the Array.Reverse method for small strings:

public static unsafe string Reverse(string text)
{
    int len = text.Length;

    // Why allocate a char[] array on the heap when you won't use it
    // outside of this method? Use the stack.
    char* reversed = stackalloc char[len];

    // Avoid bounds-checking performance penalties.
    fixed (char* str = text)
    {
        int i = 0;
        int j = i + len - 1;
        while (i < len)
        {
            reversed[i++] = str[j--];
        }
    }

    // Need to use this overload for the System.String constructor
    // as providing just the char* pointer could result in garbage
    // at the end of the string (no guarantee of null terminator).
    return new string(reversed, 0, len);
}

Here are some benchmark results.

You can see that the performance gain shrinks and then disappears against the Array.Reverse method as the strings get larger. For small- to medium-sized strings, though, it's tough to beat this method.

Solution 9

The easy and nice answer is using the Extension Method:

static class ExtentionMethodCollection
{
    public static string Inverse(this string @base)
    {
        return new string(@base.Reverse().ToArray());
    }
}

and here's the output:

string Answer = "12345".Inverse(); // = "54321"

Solution 10

Starting with .NET Core 2.1 there is a new way to reverse a string using the string.Create method.

Note that this solution does not handle Unicode combining characters etc. correctly, as "Les Mise\u0301rables" would be converted to "selbarésiM seL". See the other answers for a better solution.

public static string Reverse(string input)
{
    return string.Create<string>(input.Length, input, (chars, state) =>
    {
        state.AsSpan().CopyTo(chars);
        chars.Reverse();
    });
}

This essentially copies the characters of input to a new string and reverses the new string in-place.

Why is string.Create useful?

When we create a string from an existing array, a new internal array is allocated and the values are copied. Otherwise, it would be possible to mutate a string after its creation (in a safe environment). That is, in the following snippet we have to allocate an array of length 10 twice, one as the buffer and one as the string's internal array.

var chars = new char[10];
// set array values
var str = new string(chars);

string.Create essentially allows us to manipulate the internal array during creation time of the string. This is, we do not need a buffer anymore and can therefore avoid allocating that one char array.

Steve Gordon has written about it in more detail here. There is also an article on MSDN.

How to use string.Create?

public static string Create<TState>(int length, TState state, SpanAction<char, TState> action);

The method takes three parameters:

  1. The length of the string to create,
  2. the data you want to use to dynamically create the new string,
  3. and a delegate that creates the final string from the data, where the first parameter points to the internal char array of the new string and the second is the data (state) you passed to string.Create.

Inside the delegate we can specify how the new string is created from the data. In our case, we just copy the characters of the input string to the Span used by the new string. Then we reverse the Span and hence the whole string is reversed.

Benchmarks

To compare my proposed way of reversing a string with the accepted answer, I have written two benchmarks using BenchmarkDotNet.

public class StringExtensions
{
    public static string ReverseWithArray(string input)
    {
        var charArray = input.ToCharArray();
        Array.Reverse(charArray);
        return new string(charArray);
    }

    public static string ReverseWithStringCreate(string input)
    {
        return string.Create(input.Length, input, (chars, state) =>
        {
            state.AsSpan().CopyTo(chars);
            chars.Reverse();
        });
    }
}

[MemoryDiagnoser]
public class StringReverseBenchmarks
{
    private string input;

    [Params(10, 100, 1000)]
    public int InputLength { get; set; }


    [GlobalSetup]
    public void SetInput()
    {
        // Creates a random string of the given length
        this.input = RandomStringGenerator.GetString(InputLength);
    }

    [Benchmark(Baseline = true)]
    public string WithReverseArray() => StringExtensions.ReverseWithArray(input);

    [Benchmark]
    public string WithStringCreate() => StringExtensions.ReverseWithStringCreate(input);
}

Here are the results on my machine:

| Method           | InputLength |         Mean |      Error |    StdDev |  Gen 0 | Allocated |
| ---------------- | ----------- | -----------: | ---------: | --------: | -----: | --------: |
| WithReverseArray | 10          |    45.464 ns |  0.4836 ns | 0.4524 ns | 0.0610 |      96 B |
| WithStringCreate | 10          |    39.749 ns |  0.3206 ns | 0.2842 ns | 0.0305 |      48 B |
|                  |             |              |            |           |        |           |
| WithReverseArray | 100         |   175.162 ns |  2.8766 ns | 2.2458 ns | 0.2897 |     456 B |
| WithStringCreate | 100         |   125.284 ns |  2.4657 ns | 2.0590 ns | 0.1473 |     232 B |
|                  |             |              |            |           |        |           |
| WithReverseArray | 1000        | 1,523.544 ns |  9.8808 ns | 8.7591 ns | 2.5768 |    4056 B |
| WithStringCreate | 1000        | 1,078.957 ns | 10.2948 ns | 9.6298 ns | 1.2894 |    2032 B |

As you can see, with ReverseWithStringCreate we allocate only half of the memory used by the ReverseWithArray method.

Solution 11

If you want to play a really dangerous game, then this is by far the fastest way there is (around four times faster than the Array.Reverse method). It's an in-place reverse using pointers.

Note that I really do not recommend this for any use, ever (have a look here for some reasons why you should not use this method), but it's just interesting to see that it can be done, and that strings aren't really immutable once you turn on unsafe code.

public static unsafe string Reverse(string text)
{
    if (string.IsNullOrEmpty(text))
    {
        return text;
    }

    fixed (char* pText = text)
    {
        char* pStart = pText;
        char* pEnd = pText + text.Length - 1;
        for (int i = text.Length / 2; i >= 0; i--)
        {
            char temp = *pStart;
            *pStart++ = *pEnd;
            *pEnd-- = temp;
        }

        return text;
    }
}

Solution 12

Firstly you don't need to call ToCharArray as a string can already be indexed as a char array, so this will save you an allocation.

The next optimisation is to use a StringBuilder to prevent unnecessary allocations (as strings are immutable, concatenating them makes a copy of the string each time). To further optimise this we pre-set the length of the StringBuilder so it won't need to expand its buffer.

public string Reverse(string text)
{
    if (string.IsNullOrEmpty(text))
    {
        return text;
    }

    StringBuilder builder = new StringBuilder(text.Length);
    for (int i = text.Length - 1; i >= 0; i--)
    {
        builder.Append(text[i]);
    }

    return builder.ToString();
}

Edit: Performance Data

I tested this function and the function using Array.Reverse with the following simple program, where Reverse1 is one function and Reverse2 is the other:

static void Main(string[] args)
{
    var text = "abcdefghijklmnopqrstuvwxyz";

    // pre-jit
    text = Reverse1(text); 
    text = Reverse2(text);

    // test
    var timer1 = Stopwatch.StartNew();
    for (var i = 0; i < 10000000; i++)
    {
        text = Reverse1(text);
    }

    timer1.Stop();
    Console.WriteLine("First: {0}", timer1.ElapsedMilliseconds);

    var timer2 = Stopwatch.StartNew();
    for (var i = 0; i < 10000000; i++)
    {
        text = Reverse2(text);
    }

    timer2.Stop();
    Console.WriteLine("Second: {0}", timer2.ElapsedMilliseconds);

    Console.ReadLine();
}

It turns out that for short strings the Array.Reverse method is around twice as quick as the one above, and for longer strings the difference is even more pronounced. So given that the Array.Reverse method is both simpler and faster I'd recommend you use that rather than this one. I leave this one up here just to show that it isn't the way you should do it (much to my surprise!)

Solution 13

Try using Array.Reverse


public string Reverse(string str)
{
    char[] array = str.ToCharArray();
    Array.Reverse(array);
    return new string(array);
}

Solution 14

public static string Reverse(string input)
{
    return string.Concat(Enumerable.Reverse(input));
}

Of course you can extend string class with Reverse method

public static class StringExtensions
{
    public static string Reverse(this string input)
    {
        return string.Concat(Enumerable.Reverse(input));
    }
}

Solution 15

"Best" can depend on many things, but here are few more short alternatives ordered from fast to slow:

string s = "z̽a̎l͘g̈o̓", pattern = @"(?s).(?<=(?:.(?=.*$(?<=((\P{M}\p{C}?\p{M}*)\1?))))*)";

string s1 = string.Concat(s.Reverse());                          // "̓ög͘l̎a̽z"  

string s2 = Microsoft.VisualBasic.Strings.StrReverse(s);         // "o̓g̈l͘a̎̽z"  

string s3 = string.Concat(StringInfo.ParseCombiningCharacters(s).Reverse()
    .Select(i => StringInfo.GetNextTextElement(s, i)));          // "o̓g̈l͘a̎z̽"  

string s4 = Regex.Replace(s, pattern, "$2").Remove(s.Length);    // "o̓g̈l͘a̎z̽"  

Solution 16

Don't bother with a function, just do it in place. Note: The second line will throw an argument exception in the Immediate window of some VS versions.

string s = "Blah";
s = new string(s.ToCharArray().Reverse().ToArray()); 

Solution 17

Sorry for long post, but this might be interesting

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        public static string ReverseUsingArrayClass(string text)
        {
            char[] chars = text.ToCharArray();
            Array.Reverse(chars);
            return new string(chars);
        }

        public static string ReverseUsingCharacterBuffer(string text)
        {
            char[] charArray = new char[text.Length];
            int inputStrLength = text.Length - 1;
            for (int idx = 0; idx <= inputStrLength; idx++) 
            {
                charArray[idx] = text[inputStrLength - idx];                
            }
            return new string(charArray);
        }

        public static string ReverseUsingStringBuilder(string text)
        {
            if (string.IsNullOrEmpty(text))
            {
                return text;
            }

            StringBuilder builder = new StringBuilder(text.Length);
            for (int i = text.Length - 1; i >= 0; i--)
            {
                builder.Append(text[i]);
            }

            return builder.ToString();
        }

        private static string ReverseUsingStack(string input)
        {
            Stack<char> resultStack = new Stack<char>();
            foreach (char c in input)
            {
                resultStack.Push(c);
            }

            StringBuilder sb = new StringBuilder();
            while (resultStack.Count > 0)
            {
                sb.Append(resultStack.Pop());
            }
            return sb.ToString();
        }

        public static string ReverseUsingXOR(string text)
        {
            char[] charArray = text.ToCharArray();
            int length = text.Length - 1;
            for (int i = 0; i < length; i++, length--)
            {
                charArray[i] ^= charArray[length];
                charArray[length] ^= charArray[i];
                charArray[i] ^= charArray[length];
            }

            return new string(charArray);
        }


        static void Main(string[] args)
        {
            string testString = string.Join(";", new string[] {
                new string('a', 100), 
                new string('b', 101), 
                new string('c', 102), 
                new string('d', 103),                                                                   
            });
            int cycleCount = 100000;

            Stopwatch stopwatch = new Stopwatch();
            stopwatch.Start();
            for (int i = 0; i < cycleCount; i++) 
            {
                ReverseUsingCharacterBuffer(testString);
            }
            stopwatch.Stop();
            Console.WriteLine("ReverseUsingCharacterBuffer: " + stopwatch.ElapsedMilliseconds + "ms");

            stopwatch.Reset();
            stopwatch.Start();
            for (int i = 0; i < cycleCount; i++) 
            {
                ReverseUsingArrayClass(testString);
            }
            stopwatch.Stop();
            Console.WriteLine("ReverseUsingArrayClass: " + stopwatch.ElapsedMilliseconds + "ms");

            stopwatch.Reset();
            stopwatch.Start();
            for (int i = 0; i < cycleCount; i++) 
            {
                ReverseUsingStringBuilder(testString);
            }
            stopwatch.Stop();
            Console.WriteLine("ReverseUsingStringBuilder: " + stopwatch.ElapsedMilliseconds + "ms");

            stopwatch.Reset();
            stopwatch.Start();
            for (int i = 0; i < cycleCount; i++) 
            {
                ReverseUsingStack(testString);
            }
            stopwatch.Stop();
            Console.WriteLine("ReverseUsingStack: " + stopwatch.ElapsedMilliseconds + "ms");

            stopwatch.Reset();
            stopwatch.Start();
            for (int i = 0; i < cycleCount; i++) 
            {
                ReverseUsingXOR(testString);
            }
            stopwatch.Stop();
            Console.WriteLine("ReverseUsingXOR: " + stopwatch.ElapsedMilliseconds + "ms");            
        }
    }
}

Results:

  • ReverseUsingCharacterBuffer: 346ms
  • ReverseUsingArrayClass: 87ms
  • ReverseUsingStringBuilder: 824ms
  • ReverseUsingStack: 2086ms
  • ReverseUsingXOR: 319ms

Solution 18

public string Reverse(string input)
{
    char[] output = new char[input.Length];

    int forwards = 0;
    int backwards = input.Length - 1;

    do
    {
        output[forwards] = input[backwards];
        output[backwards] = input[forwards];
    }while(++forwards <= --backwards);

    return new String(output);
}

public string DotNetReverse(string input)
{
    char[] toReverse = input.ToCharArray();
    Array.Reverse(toReverse);
    return new String(toReverse);
}

public string NaiveReverse(string input)
{
    char[] outputArray = new char[input.Length];
    for (int i = 0; i < input.Length; i++)
    {
        outputArray[i] = input[input.Length - 1 - i];
    }

    return new String(outputArray);
}    

public string RecursiveReverse(string input)
{
    return RecursiveReverseHelper(input, 0, input.Length - 1);
}

public string RecursiveReverseHelper(string input, int startIndex , int endIndex)
{
    if (startIndex == endIndex)
    {
        return "" + input[startIndex];
    }

    if (endIndex - startIndex == 1)
    {
        return "" + input[endIndex] + input[startIndex];
    }

    return input[endIndex] + RecursiveReverseHelper(input, startIndex + 1, endIndex - 1) + input[startIndex];
}


void Main()
{
    int[] sizes = new int[] { 10, 100, 1000, 10000 };
    for(int sizeIndex = 0; sizeIndex < sizes.Length; sizeIndex++)
    {
        string holaMundo  = "";
        for(int i = 0; i < sizes[sizeIndex]; i+= 5)
        {   
            holaMundo += "ABCDE";
        }

        string.Format("\n**** For size: {0} ****\n", sizes[sizeIndex]).Dump();

        string odnuMaloh = DotNetReverse(holaMundo);

        var stopWatch = Stopwatch.StartNew();
        string result = NaiveReverse(holaMundo);
        ("Naive Ticks: " + stopWatch.ElapsedTicks).Dump();

        stopWatch.Restart();
        result = Reverse(holaMundo);
        ("Efficient linear Ticks: " + stopWatch.ElapsedTicks).Dump();

        stopWatch.Restart();
        result = RecursiveReverse(holaMundo);
        ("Recursive Ticks: " + stopWatch.ElapsedTicks).Dump();

        stopWatch.Restart();
        result = DotNetReverse(holaMundo);
        ("DotNet Reverse Ticks: " + stopWatch.ElapsedTicks).Dump();
    }
}

Output

For size: 10

Naive Ticks: 1
Efficient linear Ticks: 0
Recursive Ticks: 2
DotNet Reverse Ticks: 1

For size: 100

Naive Ticks: 2
Efficient linear Ticks: 1
Recursive Ticks: 12
DotNet Reverse Ticks: 1

For size: 1000

Naive Ticks: 5
Efficient linear Ticks: 2
Recursive Ticks: 358
DotNet Reverse Ticks: 9

For size: 10000

Naive Ticks: 32
Efficient linear Ticks: 28
Recursive Ticks: 84808
DotNet Reverse Ticks: 33

Solution 19

Simplest way:

string reversed = new string(text.Reverse().ToArray());

Solution 20

Stack-based solution.

    public static string Reverse(string text)
    {
        var stack = new Stack<char>(text);
        var array = new char[stack.Count];

        int i = 0;
        while (stack.Count != 0)
        {
            array[i++] = stack.Pop();
        }

        return new string(array);
    }

Or

    public static string Reverse(string text)
    {
        var stack = new Stack<char>(text);
        return string.Join("", stack);
    }

Solution 21

How about:

    private string Reverse(string stringToReverse)
    {
        char[] rev = stringToReverse.Reverse().ToArray();
        return new string(rev); 
    }

Solution 22

I've made a C# port from Microsoft.VisualBasic.Strings. I'm not sure why they keep such useful functions (from VB) outside the System.String in Framework, but still under Microsoft.VisualBasic. Same scenario for financial functions (e.g. Microsoft.VisualBasic.Financial.Pmt()).

public static string StrReverse(this string expression)
{
    if ((expression == null))
        return "";

    int srcIndex;

    var length = expression.Length;
    if (length == 0)
        return "";

    //CONSIDER: Get System.String to add a surrogate aware Reverse method

    //Detect if there are any graphemes that need special handling
    for (srcIndex = 0; srcIndex <= length - 1; srcIndex++)
    {
        var ch = expression[srcIndex];
        var uc = char.GetUnicodeCategory(ch);
        if (uc == UnicodeCategory.Surrogate || uc == UnicodeCategory.NonSpacingMark || uc == UnicodeCategory.SpacingCombiningMark || uc == UnicodeCategory.EnclosingMark)
        {
            //Need to use special handling
            return InternalStrReverse(expression, srcIndex, length);
        }
    }

    var chars = expression.ToCharArray();
    Array.Reverse(chars);
    return new string(chars);
}

///<remarks>This routine handles reversing Strings containing graphemes
/// GRAPHEME: a text element that is displayed as a single character</remarks>
private static string InternalStrReverse(string expression, int srcIndex, int length)
{
    //This code can only be hit one time
    var sb = new StringBuilder(length) { Length = length };

    var textEnum = StringInfo.GetTextElementEnumerator(expression, srcIndex);

    //Init enumerator position
    if (!textEnum.MoveNext())
    {
        return "";
    }

    var lastSrcIndex = 0;
    var destIndex = length - 1;

    //Copy up the first surrogate found
    while (lastSrcIndex < srcIndex)
    {
        sb[destIndex] = expression[lastSrcIndex];
        destIndex -= 1;
        lastSrcIndex += 1;
    }

    //Now iterate through the text elements and copy them to the reversed string
    var nextSrcIndex = textEnum.ElementIndex;

    while (destIndex >= 0)
    {
        srcIndex = nextSrcIndex;

        //Move to next element
        nextSrcIndex = (textEnum.MoveNext()) ? textEnum.ElementIndex : length;
        lastSrcIndex = nextSrcIndex - 1;

        while (lastSrcIndex >= srcIndex)
        {
            sb[destIndex] = expression[lastSrcIndex];
            destIndex -= 1;
            lastSrcIndex -= 1;
        }
    }

    return sb.ToString();
}

Solution 23

Had to submit a recursive example:

private static string Reverse(string str)
{
    if (str.IsNullOrEmpty(str) || str.Length == 1)
        return str;
    else
        return str[str.Length - 1] + Reverse(str.Substring(0, str.Length - 1));
}

Solution 24

Sorry for posting on this old thread. I am practicing some code for an interview.

This was what I came up with for C#. My first version before refactoring was horrible.

static String Reverse2(string str)
{
    int strLen = str.Length, elem = strLen - 1;
    char[] charA = new char[strLen];

    for (int i = 0; i < strLen; i++)
    {
        charA[elem] = str[i];
        elem--;
    }

    return new String(charA);
}

In Contrast to the Array.Reverse method below, it appears faster with 12 characters or less in the string. After 13 characters, the Array.Reverse starts to get faster, and it eventually dominates pretty heavily on speed. I just wanted to point out approximately where the speed starts to change.

static String Reverse(string str)
{     
    char[] charA = str.ToCharArray();

    Array.Reverse(charA);

    return new String(charA);
}

At 100 characters in the string, it is faster than my version x 4. However, if I knew that the strings would always be less than 13 characters, I would use the one I made.

Testing was done with Stopwatch and 5000000 iterations. Also, I'm not sure if my version handles Surrogates or combined character situations with Unicode encoding.

Solution 25

Since I like a couple of the answers - one for using string.Create and therefore high performance and low allocation and another for correctness - using the StringInfo class, I decided a combined approach is needed. This is the ultimate string reversal method :)

private static string ReverseString(string str)
    {
        return string.Create(str.Length, str, (chars, state) =>
        {
            var enumerator = StringInfo.GetTextElementEnumerator(state);
            var position = state.Length;
            while (enumerator.MoveNext())
            {
                var cluster = ((string)enumerator.Current).AsSpan();
                cluster.CopyTo(chars.Slice(position - cluster.Length));
                position -= cluster.Length;
            }
        });
    }

There's an even better way using a method of the StringInfo class which skips a lot of string allocations by the Enumerator by returning just the indexes.

private static string ReverseString(string str)
    {
        return string.Create(str.Length, str, (chars, state) =>
        {
            var position = 0;
            var indexes = StringInfo.ParseCombiningCharacters(state); // skips string creation
            var stateSpan = state.AsSpan();
            for (int len = indexes.Length, i = len - 1; i >= 0; i--)
            {
                var index = indexes[i];
                var spanLength = i == len - 1 ? state.Length - index : indexes[i + 1] - index;
                stateSpan.Slice(index, spanLength).CopyTo(chars.Slice(position));
                position += spanLength;
            }
        });
    }

Some benchmarks compared to the LINQ solution:

String length 20:

LINQ                       Mean: 2,355.5 ns   Allocated: 1440 B
string.Create              Mean:   851.0 ns   Allocated:  720 B
string.Create with indexes Mean:   466.4 ns   Allocated:  168 B

String length 450:

LINQ                          Mean: 34.33 us   Allocated: 22.98 KB
string.Create                 Mean:   19.13 us   Allocated: 14.98 KB
string.Create with indexes    Mean:   10.32 us   Allocated: 2.69 KB

Solution 26

"Better way" depends on what is more important to you in your situation, performance, elegance, maintainability etc.

Anyway, here's an approach using Array.Reverse:

string inputString="The quick brown fox jumps over the lazy dog.";
char[] charArray = inputString.ToCharArray(); 
Array.Reverse(charArray); 

string reversed = new string(charArray);

Solution 27

If it ever came up in an interview and you were told you can't use Array.Reverse, i think this might be one of the fastest. It does not create new strings and iterates only over half of the array (i.e O(n/2) iterations)

    public static string ReverseString(string stringToReverse)
    {
        char[] charArray = stringToReverse.ToCharArray();
        int len = charArray.Length-1;
        int mid = len / 2;

        for (int i = 0; i < mid; i++)
        {
            char tmp = charArray[i];
            charArray[i] = charArray[len - i];
            charArray[len - i] = tmp;
        }
        return new string(charArray);
    }

Solution 28

If you have a string that only contains ASCII characters, you can use this method.

    public static string ASCIIReverse(string s)
    {
        byte[] reversed = new byte[s.Length];

        int k = 0;
        for (int i = s.Length - 1; i >= 0; i--)
        {
            reversed[k++] = (byte)s[i];
        }

        return Encoding.ASCII.GetString(reversed);
    }

Solution 29

First of all what you have to understand is that str+= will resize your string memory to make space for 1 extra char. This is fine, but if you have, say, a book with 1000 pages that you want to reverse, this will take very long to execute.

The solution that some people might suggest is using StringBuilder. What string builder does when you perform a += is that it allocates much larger chunks of memory to hold the new character so that it does not need to do a reallocation every time you add a char.

If you really want a fast and minimal solution I'd suggest the following:

            char[] chars = new char[str.Length];
            for (int i = str.Length - 1, j = 0; i >= 0; --i, ++j)
            {
                chars[j] = str[i];
            }
            str = new String(chars);

In this solution there is one initial memory allocation when the char[] is initialized and one allocation when the string constructor builds the string from the char array.

On my system I ran a test for you that reverses a string of 2 750 000 characters. Here are the results for 10 executions:

StringBuilder: 190K - 200K ticks

Char Array: 130K - 160K ticks

I also ran a test for normal String += but I abandoned it after 10 minutes with no output.

However, I also noticed that for smaller strings the StringBuilder is faster, so you will have to decide on the implementation based on the input.

Cheers

Solution 30

public static string reverse(string s) 
{
    string r = "";
    for (int i = s.Length; i > 0; i--) r += s[i - 1];
    return r;
}