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
//Replace blank " " with "%20"//Note - the 'length' parameter is the maximum length of the array
void ReplaceBlanks(char str[], intlength)
{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." <<:endl>
}//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;
}
}voidunitest()
{char s[100] = "We are happy.";
std::cout<< "Before replacing blanks, the string is" << s <<:endl>
ReplaceBlanks(s,100);
std::cout<< "After replacing blanks, the string is" << s <<:endl>
}intmain()
{
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.
defreplace_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_countif 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 - 1new_index= replaced_length - 1
while new_index !=original_index:if string[original_index] == ' ':
new_index-= 2string[new_index:new_index+3] = '%20'
else:
string[new_index]=string[original_index]#Update indexes
new_index -= 1original_index-= 1
defunitest():
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// isthe 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 ifyou have to target a platform prior to the ones specified below.// Refer to MSDN for the latest info on corresponding values fordifferent 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#include
//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
/*length 为字符数组string的总容量*/
void ReplaceBlank(char string[], intlength)
{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, charexpected[])
{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");elseprintf("failed.\n");
}//空格在句子中间
voidTest1()
{const int length = 100;char string[length] = "hello world";
Test("Test1", string, length, "hello%20world");
}//空格在句子开头
voidTest2()
{const int length = 100;char string[length] = "helloworld";
Test("Test2", string, length, "%20helloworld");
}//空格在句子末尾
voidTest3()
{const int length = 100;char string[length] = "helloworld";
Test("Test3", string, length, "helloworld%20");
}//连续有两个空格
voidTest4()
{const int length = 100;char string[length] = "hello world";
Test("Test4", string, length, "hello%20%20world");
}//传入NULL
voidTest5()
{
Test("Test5", NULL, 0, NULL);
}//传入内容为空的字符串
voidTest6()
{const int length = 100;char string[length] = "";
Test("Test6", string, length, "");
}//传入内容为一个空格的字符串
voidTest7()
{const int length = 100;char string[length] = " ";
Test("Test7", string, length, "%20");
}//传入的字符串没有空格
voidTest8()
{const int length = 100;char string[length] = "helloworld";
Test("Test8", string, length, "helloworld");
}//传入的字符串全是空格
voidTest9()
{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
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.
