字符串反转的N种方法

字符串反转的N种方法

数组反转。这的确是一道非常基础的算法题，然而也是一道很不平常的算法题（也许所有的算法深究下去都会很不平常）。因为我写着写着，就写出来8种方法……现在我们以字符串的反转为例，来介绍这几种方法并对它们的性能进行比较。

使用Array.Reverse方法

对于字符串反转，我们可以使用.NET类库自带的Array.Reverse方法

public static string  ReverseByArray(this string  original)

{

    char[] c = original.ToCharArray();

    Array.Reverse(c);

    return new string(c);

}

使用字符缓存

在面试或笔试中，往往要求不用任何类库方法，那么有朋友大概会使用类似下面这样的循环方法

public static string ReverseByCharBuffer(this string original)

{

    char[] c = original.ToCharArray();

    int l = original.Length;

    char[] o = new char[l];

    for (int i = 0; i < l ; i++)

    {

        o[i] = c[l - i - 1];

    }

    return new string(o);

}

当然，聪明的同学们一定会发现不必对这个字符数组进行完全遍历，通常情况下我们会只遍历一半

public static string ReverseByCharBuffer2(this string original)

{

    char[] c = original.ToCharArray();

    int l = original.Length;

    for (int i = 0; i < l / 2; i++)

    {

        char t = c[i];

        c[i] = c[l - i - 1];

        c[l - i - 1] = t;

    }

    return new string(c);

}

ReverseByCharBuffer使用了一个新的数组，而且遍历了字符数组的所有元素，因此时间和空间的开销都要大于ReverseByCharBuffer2。

在Array.Reverse内部，调用了非托管方法TrySZReverse，如果TrySZReverse不成功，实际上也是调用了类似ReverseByCharBuffer2的方法。

if (!TrySZReverse(array, index, length))

{

    int num = index;

    int num2 = (index + length) - 1;

    object[] objArray = array as object[];

    if (objArray == null)

    {

        while (num < num2)

        {

            object obj3 = array.GetValue(num);

            array.SetValue(array.GetValue(num2), num);

            array.SetValue(obj3, num2);

            num++;

            num2--;

        }

    }

    else

    {

        while (num < num2)

        {

            object obj2 = objArray[num];

            objArray[num] = objArray[num2];

            objArray[num2] = obj2;

            num++;

            num2--;

        }

    }

}

大致上我能想到的算法就是这么多了，但是我无意间发现了StackOverflow上的一篇帖子，才发现这么一个看似简单的反转算法实现起来真可谓花样繁多。

使用StringBuilder

使用StringBuilder方法大致和ReverseByCharBuffer一样，只不过不使用字符数组做缓存，而是使用StringBuilder。

public static string ReverseByStringBuilder(this string original)

{

    StringBuilder sb = new StringBuilder(original.Length);

    for (int i = original.Length - 1; i >= 0; i--)

    {

        sb.Append(original[i]);

    }

    return sb.ToString();

}

当然，你可以预见，这种算法的效率不会比ReverseByCharBuffer要高。

我们可以像使用字符缓存那样，对使用StringBuilder方法进行优化，使其遍历过程也减少一半

public static string ReverseByStringBuilder2(this string original)

{

    StringBuilder sb = new StringBuilder(original);

    for (int i = 0, j = original.Length - 1; i <= j; i++, j--)

    {

        sb[i] = original[j];

        sb[j] = original[i];

    }

    return sb.ToString();

}

以上这几种方法按算法角度来说，其实可以归结为一类。然而下面的几种算法就完全不是同一类型的了。

使用栈

栈是一个很神奇的数据结构。我们可以使用它后进先出的特性来对数组进行反转。先将数组所有元素压入栈，然后再取出，顺序很自然地就与原先相反了。

public static string ReverseByStack(this string original)

{

    Stack<char> stack = new Stack<char>();

    foreach (char ch in original)

    {

        stack.Push(ch);

    }

    char[] c = new char[original.Length];

    for (int i = 0; i < original.Length; i++)

    {

        c[i] = stack.Pop();

    }

    return new string(c);

}

两次循环和栈的开销无疑使这种方法成为目前为止开销最大的方法。但使用栈这个数据结构的想法还是非常有价值的。

使用XOR

使用逻辑异或也可以进行反转

public static string ReverseByXor(this string original)

{

    char[] charArray = original.ToCharArray();

    int l = original.Length - 1;

    for (int i = 0; i < l; i++, l--)

    {

        charArray[i] ^= charArray[l];

        charArray[l] ^= charArray[i];

        charArray[i] ^= charArray[l];

    }

    return new string(charArray);

}

在C#中，x ^= y相当于x = x ^ y。通过3次异或操作，可以将两个字符为止互换。对于算法具体的解释可以参考这篇文章。

使用指针

使用指针可以达到最快的速度，但是unsafe代码不是微软所推荐的，在这里我们就不多做讨论了

public static unsafe string ReverseByPointer(this string original)

{

    fixed (char* pText = original)

    {

        char* pStart = pText;

        char* pEnd = pText + original.Length - 1;

        for (int i = original.Length / 2; i >= 0; i--)

        {

            char temp = *pStart;

            *pStart++ = *pEnd;

            *pEnd-- = temp;

        }



        return original;

    }

}

使用递归

对于反转这类算法，都可以使用递归方法

public static string ReverseByRecursive(this string original)

{

    if (original.Length == 1)

        return original;

    else

        return original.Substring(1).ReverseByRecursive() + original[0];

}

使用委托，还可以使代码变得更加简洁

public static string ReverseByRecursive2(this string original)

{

    Func<string, string> f = null;

    f = s => s.Length > 0 ? f(s.Substring(1)) + s[0] : string.Empty;

    return f(original);

}

但是委托开销大的弊病在这里一点也没有减少，以至于我做性能测试的时候导致系统假死甚至内存益处。

使用LINQ

System.Enumerable里提供了默认的Reverse扩展方法，我们可以基于该方法来对String类型进行扩展

public static string ReverseByLinq(this string original)

{

    return new string(original.Reverse().ToArray());

}

性能比较

接下来让我们来对以上8种方法的11个扩展方法来进行性能比较。

影响字符串反转算法性能的因素主要就是字符串的长度。让我们分别取1、10、15、25、50、75、100、1000、10000作为字符串长度来进行测试。用下面的方法来随机生成不同长度的字符串

static string GenerateStringByLength(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();

}

用下面的方法来计算时间

static void Benchmark(string description, Func<string> func, int times)

{

    Stopwatch sw = new Stopwatch();

    sw.Start();

    for (int j = 0; j < times; j++)

    {

        func();

    }

    sw.Stop();

    Debug.WriteLine("{0} Ticks {1} : called {2} times.", 

        sw.ElapsedTicks, description, times);

}

测试的主方法如下

static void Main(string[] args)

{

    // 预热

    "abcde".ReverseByArray();

    "abcde".ReverseByCharBuffer();

    "abcde".ReverseByCharBuffer2();

    "abcde".ReverseByLinq();

    "abcde".ReverseByPointer();

    "abcde".ReverseByRecursive();

    "abcde".ReverseByRecursive2();

    "abcde".ReverseByStack();

    "abcde".ReverseByStringBuilder();

    "abcde".ReverseByStringBuilder2();

    "abcde".ReverseByXor();



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



    foreach (int l in lengths)

    {

        int iterations = 100;

        string text = GenerateStringByLength(l);

        Benchmark(String.Format("ReverseByArray (Length: {0})", l),

            text.ReverseByArray, iterations);

        Benchmark(String.Format("ReverseByCharBuffer (Length: {0})", l), 

            text.ReverseByCharBuffer, iterations);

        Benchmark(String.Format("ReverseByCharBuffer2 (Length: {0})", l), 

            text.ReverseByCharBuffer2, iterations);

        Benchmark(String.Format("ReverseByStringBuilder (Length: {0})", l),

            text.ReverseByStringBuilder, iterations);

        Benchmark(String.Format("ReverseByStringBuilder2 (Length: {0})", l),

            text.ReverseByStringBuilder2, iterations);

        Benchmark(String.Format("ReverseByStack (Length: {0})", l), 

            text.ReverseByStack, iterations);

        Benchmark(String.Format("ReverseByXor (Length: {0})", l), 

            text.ReverseByXor, iterations);

        Benchmark(String.Format("ReverseByPointer (Length: {0})", l), 

            text.ReverseByPointer, iterations);

        Benchmark(String.Format("ReverseByRecursive (Length: {0})", l), 

            text.ReverseByRecursive, iterations);

        Benchmark(String.Format("ReverseByRecursive2 (Length: {0})", l), 

            text.ReverseByRecursive2, iterations);

        Benchmark(String.Format("ReverseByLinq (Length: {0})", l), 

            text.ReverseByLinq, iterations);



        Debug.WriteLine(Environment.NewLine);

    }

}

好了，来看看结果吧。（由于递归算法与其他算法的开销不在一个数量级上，因此忽略了对该算法的比较）

197602 Ticks ReverseByArray (Length: 1) : called 100 times.

75773 Ticks ReverseByCharBuffer (Length: 1) : called 100 times.

111833 Ticks ReverseByCharBuffer2 (Length: 1) : called 100 times.

134535 Ticks ReverseByStringBuilder (Length: 1) : called 100 times.

148598 Ticks ReverseByStringBuilder2 (Length: 1) : called 100 times.

192435 Ticks ReverseByStack (Length: 1) : called 100 times.

63098 Ticks ReverseByXor (Length: 1) : called 100 times.

51945 Ticks ReverseByPointer (Length: 1) : called 100 times.

587865 Ticks ReverseByLinq (Length: 1) : called 100 times.





185325 Ticks ReverseByArray (Length: 10) : called 100 times.

189712 Ticks ReverseByCharBuffer (Length: 10) : called 100 times.

100155 Ticks ReverseByCharBuffer2 (Length: 10) : called 100 times.

216232 Ticks ReverseByStringBuilder (Length: 10) : called 100 times.

209497 Ticks ReverseByStringBuilder2 (Length: 10) : called 100 times.

669832 Ticks ReverseByStack (Length: 10) : called 100 times.

163237 Ticks ReverseByXor (Length: 10) : called 100 times.

74303 Ticks ReverseByPointer (Length: 10) : called 100 times.

1058348 Ticks ReverseByLinq (Length: 10) : called 100 times.





215437 Ticks ReverseByArray (Length: 15) : called 100 times.

206610 Ticks ReverseByCharBuffer (Length: 15) : called 100 times.

168180 Ticks ReverseByCharBuffer2 (Length: 15) : called 100 times.

260542 Ticks ReverseByStringBuilder (Length: 15) : called 100 times.

296153 Ticks ReverseByStringBuilder2 (Length: 15) : called 100 times.

785857 Ticks ReverseByStack (Length: 15) : called 100 times.

177915 Ticks ReverseByXor (Length: 15) : called 100 times.

84802 Ticks ReverseByPointer (Length: 15) : called 100 times.

1113262 Ticks ReverseByLinq (Length: 15) : called 100 times.





266167 Ticks ReverseByArray (Length: 25) : called 100 times.

260820 Ticks ReverseByCharBuffer (Length: 25) : called 100 times.

236025 Ticks ReverseByCharBuffer2 (Length: 25) : called 100 times.

380408 Ticks ReverseByStringBuilder (Length: 25) : called 100 times.

440430 Ticks ReverseByStringBuilder2 (Length: 25) : called 100 times.

1197593 Ticks ReverseByStack (Length: 25) : called 100 times.

262388 Ticks ReverseByXor (Length: 25) : called 100 times.

110453 Ticks ReverseByPointer (Length: 25) : called 100 times.

1611900 Ticks ReverseByLinq (Length: 25) : called 100 times.





258435 Ticks ReverseByArray (Length: 50) : called 100 times.

474135 Ticks ReverseByCharBuffer (Length: 50) : called 100 times.

341655 Ticks ReverseByCharBuffer2 (Length: 50) : called 100 times.

662242 Ticks ReverseByStringBuilder (Length: 50) : called 100 times.

587078 Ticks ReverseByStringBuilder2 (Length: 50) : called 100 times.

2116350 Ticks ReverseByStack (Length: 50) : called 100 times.

417375 Ticks ReverseByXor (Length: 50) : called 100 times.

177847 Ticks ReverseByPointer (Length: 50) : called 100 times.

9114592 Ticks ReverseByLinq (Length: 50) : called 100 times.





270022 Ticks ReverseByArray (Length: 75) : called 100 times.

488647 Ticks ReverseByCharBuffer (Length: 75) : called 100 times.

378225 Ticks ReverseByCharBuffer2 (Length: 75) : called 100 times.

1096148 Ticks ReverseByStringBuilder (Length: 75) : called 100 times.

772312 Ticks ReverseByStringBuilder2 (Length: 75) : called 100 times.

3069427 Ticks ReverseByStack (Length: 75) : called 100 times.

479092 Ticks ReverseByXor (Length: 75) : called 100 times.

234195 Ticks ReverseByPointer (Length: 75) : called 100 times.

3330945 Ticks ReverseByLinq (Length: 75) : called 100 times.





319717 Ticks ReverseByArray (Length: 100) : called 100 times.

584857 Ticks ReverseByCharBuffer (Length: 100) : called 100 times.

505470 Ticks ReverseByCharBuffer2 (Length: 100) : called 100 times.

1076715 Ticks ReverseByStringBuilder (Length: 100) : called 100 times.

942375 Ticks ReverseByStringBuilder2 (Length: 100) : called 100 times.

4390493 Ticks ReverseByStack (Length: 100) : called 100 times.

649725 Ticks ReverseByXor (Length: 100) : called 100 times.

293025 Ticks ReverseByPointer (Length: 100) : called 100 times.

6405082 Ticks ReverseByLinq (Length: 100) : called 100 times.





3262087 Ticks ReverseByArray (Length: 1000) : called 100 times.

5511607 Ticks ReverseByCharBuffer (Length: 1000) : called 100 times.

9097485 Ticks ReverseByCharBuffer2 (Length: 1000) : called 100 times.

10325760 Ticks ReverseByStringBuilder (Length: 1000) : called 100 times.

18120420 Ticks ReverseByStringBuilder2 (Length: 1000) : called 100 times.

40247490 Ticks ReverseByStack (Length: 1000) : called 100 times.

6837915 Ticks ReverseByXor (Length: 1000) : called 100 times.

2654011 Ticks ReverseByPointer (Length: 1000) : called 100 times.

84809355 Ticks ReverseByLinq (Length: 1000) : called 100 times.





368229982 Ticks ReverseByArray (Length: 100000) : called 100 times.

609454380 Ticks ReverseByCharBuffer (Length: 100000) : called 100 times.

507932685 Ticks ReverseByCharBuffer2 (Length: 100000) : called 100 times.

748738972 Ticks ReverseByStringBuilder (Length: 100000) : called 100 times.

732820133 Ticks ReverseByStringBuilder2 (Length: 100000) : called 100 times.

2249140177 Ticks ReverseByStack (Length: 100000) : called 100 times.

508241490 Ticks ReverseByXor (Length: 100000) : called 100 times.

192039592 Ticks ReverseByPointer (Length: 100000) : called 100 times.

2346782325 Ticks ReverseByLinq (Length: 100000) : called 100 times.

整理成表格如下

 

 

绘制成更直观的折线图（由于数量级差别太大，故舍去1000和10000长度的情况）

 

是的，LINQ方法处理长度为50的数组时，效率比长度为75的数组还要低。我测试了很多次，都是这样的结果，感兴趣的朋友可以深入研究一下。

将耗时明显偏高的LINQ方法和Stack方法去掉，剩下各种算法在时间上的优劣就一目了然了。

可见，直接使用指针的效率是最高的。而类库自带的Array.Reverse尽管在面对长度较小的数组时没有明显优势，但面对大数组其算法效率却十分稳定。XOR方法在小数组面前效率很高，但面对大数组就败下阵来。遍历了数组一半元素的CharBuffer2表现优异，无论面对大数组还是小数组，排名都很靠前。

指针方法尽管高效，但其带来的问题也许会很严重，而且面对大数组时Array.Reverse也同样优秀，因此一般情况下还是推荐使用Array.Reverse。当然如果面试官希望你拿出一套不使用类库的高效方案，CharBuffer2将是最佳选择。

当然，你也可以去找一个数组长度的临界点，在临界点以下使用CharBuffer2，在临界点以上使用Array.Reverse。如

public static string Reverse(this string original)

{

    if (original.Length <= 25)

        return original.ReverseByCharBuffer2();

    else

        return original.ReverseByArray();

}

希望本文对你有所帮助。