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 <= 0) // 易漏点 return; // Count the number of blanks
    char *pChar = str; int strLen = 0; int blanksCount = 0; 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 + 2 * blanksCount; if (replacedStrLen > length) { std::cout << "The char array is lack of space." << std::endl; return; } // Char pointer initialization
    char *pChar2 = str + replacedStrLen - 1; pChar = str + strLen - 1; while (pChar != pChar2) { // Replace blanks with "%20"
        if (*pChar == ' ') { pChar2 -= 2; *pChar2 = '%'; *(pChar2 + 1) = '2'; *(pChar2 + 2) = '0'; } else { *pChar2 = *pChar; } --pChar; --pChar2; } } void unitest() { char s[100] = "We are happy."; std::cout << "Before replacing blanks, the string is " << s << std::endl; ReplaceBlanks(s, 100); std::cout << "After replacing blanks, the string is " << s << std::endl; } int main() { unitest(); return 0; }

 

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 <= 0) return; /*originalLength 为字符串string的实际长度*/
    int originalLength = 0; int numberOfBlank = 0; int i = 0; 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 * 2; if(newLength > length) return; int indexOfOriginal = originalLength; int indexOfNew = newLength; while(indexOfOriginal >= 0 && indexOfNew > indexOfOriginal) { if(string[indexOfOriginal] == ' ') { string[indexOfNew --] = '0'; string[indexOfNew --] = '2'; 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) == 0) printf("passed.\n"); else printf("failed.\n"); } // 空格在句子中间
void Test1() { const int length = 100; char string[length] = "hello world"; Test("Test1", string, length, "hello%20world"); } // 空格在句子开头
void Test2() { const int length = 100; char string[length] = " helloworld"; Test("Test2", string, length, "%20helloworld"); } // 空格在句子末尾
void Test3() { const int length = 100; char string[length] = "helloworld "; Test("Test3", string, length, "helloworld%20"); } // 连续有两个空格
void Test4() { const int length = 100; char string[length] = "hello world"; Test("Test4", string, length, "hello%20%20world"); } // 传入NULL
void Test5() { Test("Test5", NULL, 0, NULL); } // 传入内容为空的字符串
void Test6() { const int length = 100; 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 = 100; char string[length] = " "; Test("Test7", string, length, "%20"); } // 传入的字符串没有空格
void Test8() { const int length = 100; char string[length] = "helloworld"; Test("Test8", string, length, "helloworld"); } // 传入的字符串全是空格
void Test9() { const int length = 100; 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 0; }

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.