替换空格（C++和Python 实现）

（说明：本博客中的题目、题目详细说明及参考代码均摘自 “何海涛《剑指Offer：名企面试官精讲典型编程题》2012年”）

题目

请实现一个函数，把字符串中的每个空格替换为 "%20" 。例如输入 "We are happy.", 则输出 "We%20are%20happy." 。

进一步详细说明：

在网络编程中，如果 URL 参数中含有特殊字符，如空格、'#'、':' 等，可能导致服务器端无法获得正确的参数值。我们需要这些特殊符号转换成服务器可以识别的字符。转换规则是在 '%' 后面跟上 ASCII 码的两位十六进制的表示。如空格的 ASCII 码是 32，即十六进制的 0x20，因此空格被替换成 "%20" 。再比如 '#' 的 ASCII 码为 35，即十六进制的 0x23，它在 URL 中被替换为 "%32"。再比如 ':' 的 ASCII 码为 50，即十六进制的 0x32，它在 URL 中被替换为 "%32"。

算法设计思想

1. 时间复杂度为 O(n²) 的算法思想

从头到尾，扫描字符串中的每个字符，遇到空格，先将剩余的字符（未遍历到的字符串）整体向后移动2个位置，然后，在空格和其后的2个字符的替换为"%20"。

2. 时间复杂度为 O(n) 的算法思想

先遍历整个字符串，计算字符串中空格的总数，从而可以计算出替换后的字符串长度（根据替换规则，每次替换空格时，都会使字符串的长度增加2）。然后，使用两个指针或索引，从后往前遍历，即初始化指针或索引分别指向替换前和替换后字符串的末尾，循环递减，如遇到空格，则替换为 "%20"，从而减少字符串移动的次数，降低时间复杂度。

C++ 实现

#include <iostream>

// Replace blank " " with "%20"
// Note - the 'length' parameter is the maximum length of the array
void ReplaceBlanks(char str[], int length)
{
    if (str == NULL || length <= )  // 易漏点
        return;

    // Count the number of blanks
    char *pChar = str;
    int strLen = ;
    int blanksCount = ;
    while (*pChar++ != '\0') {   // 易错点，容易漏掉对指针的递增操作，而导致运行时的死循环。
        ++strLen;
        if (*pChar == ' ')
            blanksCount++;
    }
    // Compute the replaced string length
    int replacedStrLen = strLen +  * blanksCount;
    if (replacedStrLen > length) {
        std::cout << "The char array is lack of space." << std::endl;
        return;
    }
    // Char pointer initialization
    char *pChar2 = str + replacedStrLen - ;
    pChar = str + strLen - ;
    while (pChar != pChar2) {
        // Replace blanks with "%20"
        if (*pChar == ' ') {
            pChar2 -= ;
            *pChar2 = '%';
            *(pChar2 + ) = '';
            *(pChar2 + ) = '';
        } else {
            *pChar2 = *pChar;
        }
        --pChar;
        --pChar2;
    }
}

void unitest()
{
    char s[] = "We are happy.";
    std::cout << "Before replacing blanks, the string is " << s << std::endl;
    ReplaceBlanks(s, );
    std::cout << "After replacing blanks, the string is " << s << std::endl;
}

int main()
{
    unitest();

    return ;
}

Python 实现

#!/usr/bin/python
# -*- coding: utf8 -*-

# Replace blank " " with "%20"
# Note, the 'string' parameter is Python list type;
#   and the 'length' parameter is the maximum length of the array.
def replace_blanks(string, length):
    if string == None or length <= 0:  # 易漏点
        return

    # Count the number of blanks
    blanks_count = string.count(' ')
    string_length = len(string)

    # Compute the replaced string length
    replaced_length = string_length + 2 * blanks_count
    if replaced_length > length:
        return
    # Extend the char list length 'string_length' with '' characters
    string += ["" for i in range(replaced_length - string_length)]

    # Replace each blank with "%20"
    original_index = string_length - 1
    new_index = replaced_length - 1
    while new_index != original_index:
        if string[original_index] == ' ':
            new_index -= 2
            string[new_index:new_index+3] = '%20'
        else:
            string[new_index] = string[original_index]
        # Update indexes
        new_index -= 1
        original_index -= 1

def unitest():
    test_string = "We are happy."
    string_lst = list(test_string)  # 易错点，不能用'str'对象替代，因为 'str' object does not support item assignment 。
    print "Before replacing blanks, the string is %s" % ''.join(string_lst)
    replace_blanks(string_lst, 100)
    print "After replacing blanks, the string is %s" % ''.join(string_lst)

if __name__ == '__main__':
    unitest()

参考代码

1. targetver.h

#pragma once

// The following macros define the minimum required platform.  The minimum required platform
// is the earliest version of Windows, Internet Explorer etc. that has the necessary features to run
// your application.  The macros work by enabling all features available on platform versions up to and
// including the version specified.

// Modify the following defines if you have to target a platform prior to the ones specified below.
// Refer to MSDN for the latest info on corresponding values for different platforms.
#ifndef _WIN32_WINNT            // Specifies that the minimum required platform is Windows Vista.
#define _WIN32_WINNT 0x0600     // Change this to the appropriate value to target other versions of Windows.
#endif

2. stdafx.h

// stdafx.h : include file for standard system include files,
// or project specific include files that are used frequently, but
// are changed infrequently
//

#pragma once

#include "targetver.h"

#include <stdio.h>
#include <tchar.h>

// TODO: reference additional headers your program requires here

3. stdafx.cpp

// stdafx.cpp : source file that includes just the standard includes
// ReplaceBlank.pch will be the pre-compiled header
// stdafx.obj will contain the pre-compiled type information

#include "stdafx.h"

// TODO: reference any additional headers you need in STDAFX.H
// and not in this file

4. ReplaceBlank.cpp

// ReplaceBlank.cpp : Defines the entry point for the console application.
//

// 《剑指Offer——名企面试官精讲典型编程题》代码
// 著作权所有者：何海涛

#include "stdafx.h"
#include <string>

/*length 为字符数组string的总容量*/
void ReplaceBlank(char string[], int length)
{
    if(string == NULL && length <= )
        return;

    /*originalLength 为字符串string的实际长度*/
    int originalLength = ;
    int numberOfBlank = ;
    int i = ;
    while(string[i] != '\0')
    {
        ++ originalLength;

        if(string[i] == ' ')
            ++ numberOfBlank;

        ++ i;
    }

    /*newLength 为把空格替换成'%20'之后的长度*/
    int newLength = originalLength + numberOfBlank * ;
    if(newLength > length)
        return;

    int indexOfOriginal = originalLength;
    int indexOfNew = newLength;
    while(indexOfOriginal >=  && indexOfNew > indexOfOriginal)
    {
        if(string[indexOfOriginal] == ' ')
        {
            string[indexOfNew --] = '';
            string[indexOfNew --] = '';
            string[indexOfNew --] = '%';
        }
        else
        {
            string[indexOfNew --] = string[indexOfOriginal];
        }

        -- indexOfOriginal;
    }
}

void Test(char* testName, char string[], int length, char expected[])
{
    if(testName != NULL)
        printf("%s begins: ", testName);

    ReplaceBlank(string, length);

    if(expected == NULL && string == NULL)
        printf("passed.\n");
    else if(expected == NULL && string != NULL)
        printf("failed.\n");
    else if(strcmp(string, expected) == )
        printf("passed.\n");
    else
        printf("failed.\n");
}

// 空格在句子中间
void Test1()
{
    const int length = ;

    char string[length] = "hello world";
    Test("Test1", string, length, "hello%20world");
}

// 空格在句子开头
void Test2()
{
    const int length = ;

    char string[length] = " helloworld";
    Test("Test2", string, length, "%20helloworld");
}

// 空格在句子末尾
void Test3()
{
    const int length = ;

    char string[length] = "helloworld ";
    Test("Test3", string, length, "helloworld%20");
}

// 连续有两个空格
void Test4()
{
    const int length = ;

    char string[length] = "hello  world";
    Test("Test4", string, length, "hello%20%20world");
}

// 传入NULL
void Test5()
{
    Test("Test5", NULL, , NULL);
}

// 传入内容为空的字符串
void Test6()
{
    const int length = ;

    char string[length] = "";
    Test("Test6", string, length, "");
}

//传入内容为一个空格的字符串
void Test7()
{
    const int length = ;

    char string[length] = " ";
    Test("Test7", string, length, "%20");
}

// 传入的字符串没有空格
void Test8()
{
    const int length = ;

    char string[length] = "helloworld";
    Test("Test8", string, length, "helloworld");
}

// 传入的字符串全是空格
void Test9()
{
    const int length = ;

    char string[length] = "   ";
    Test("Test9", string, length, "%20%20%20");
}

int _tmain(int argc, _TCHAR* argv[])
{
    Test1();
    Test2();
    Test3();
    Test4();
    Test5();
    Test6();
    Test7();
    Test8();
    Test9();

    return ;
}

5. 项目 04_ReplaceBlank 下载

百度网盘： 04_ReplaceBlank.zip

参考资料

[1]  何海涛. 剑指 Offer：名企面试官精讲典型编程题 [M]. 北京：电子工业出版社，2012. 44-48.