C++字符串空格替换题

在网络编程中，需要将URL参数中含有的特殊字符通过在‘%‘后加上ASCII码的两位十六进制的方法，转换成服务器端能够识别的字符，如空格的ASCII码为32即16进制的0x20，则需要替换为"%20"。

题目：请实现一个函数，把传入char*字符串中的每个空格替换成"%20"，例如输入"We are happy."，则输出"We%20are%20happy."

时间复杂度为O(n)的解法思路：

首先遍历字符串，得到字符串长度originalLength并统计空格数numberOfBlank，可计算出替换后的字符串长度newLength = originalLength+2*numberOfBlank；

从字符串的尾部开始复制并向前移动，用两个下标IndexOfOriginal和IndexOfNew分别标记目前所指的原字符位置和替换后的字符位置；

当IndexOfOriginal指向空格时，IndexOfNew下标所指的字符及其往前1、2个位置分别替换为‘0‘、‘2‘、‘%‘，替换完成后IndexOfOriginal向前移动一格，IndexOfNew相应向前移动3格，否则都往前移动一格；

最后在字符串尾部即下标为newLength-1的位置添加‘\0‘。

代码：

void ReplaceBlank(char str[], int maxLength) {
    if(str == NULL || maxLength <=0)
        return;

    //calculate the new length after replacing and number of Blanks
    int originalLength = 0;
    int numberOfBlank = 0;
    int index = 0;
    for(index = 0; str[index] != ‘\0‘; index++) {
        if(str[index] == ‘ ‘)
            ++numberOfBlank;
    }
    originalLength = index+1;

    //if the number of blank is 0, nothing changes
    if(numberOfBlank==0)
        return;

    //if the new length exceeds the max length
    int newLength = originalLength+2*numberOfBlank;
    if (newLength > maxLength) {
        cout << "new Length exceeds the max Length!" << endl;
        return;
    }
    //start at the character before ‘\0‘
    int indexOfOriginal = originalLength-2;
    int indexOfNew = newLength-2;
    while (indexOfOriginal>=0 && indexOfNew > indexOfOriginal) {
        if (str[indexOfOriginal] == ‘ ‘) {
            str[indexOfNew--] = ‘0‘;
            str[indexOfNew--] = ‘2‘;
            str[indexOfNew--] = ‘%‘;
        } else {
            str[indexOfNew--] = str[indexOfOriginal];
        }
        --indexOfOriginal;
    }
    //add a ‘\0‘ at last
    str[newLength-1] = ‘\0‘;
}

完整代码：

#include <iostream>
#include <stdio.h>
using namespace std;

void ReplaceBlank(char str[], int maxLength) {
    if(str == NULL || maxLength <=0)
        return;

    //calculate the new length after replacing and number of Blanks
    int originalLength = 0;
    int numberOfBlank = 0;
    int index = 0;
    for(index = 0; str[index] != ‘\0‘; index++) {
        if(str[index] == ‘ ‘)
            ++numberOfBlank;
    }
    originalLength = index+1;

    //if the number of blank is 0, nothing changes
    if(numberOfBlank==0)
        return;

    //if the new length exceeds the max length
    int newLength = originalLength+2*numberOfBlank;
    if (newLength > maxLength) {
        cout << "new Length exceeds the max Length!" << endl;
        return;
    }
    //start at the character before ‘\0‘
    int indexOfOriginal = originalLength-2;
    int indexOfNew = newLength-2;
    while (indexOfOriginal>=0 && indexOfNew > indexOfOriginal) {
        if (str[indexOfOriginal] == ‘ ‘) {
            str[indexOfNew--] = ‘0‘;
            str[indexOfNew--] = ‘2‘;
            str[indexOfNew--] = ‘%‘;
        } else {
            str[indexOfNew--] = str[indexOfOriginal];
        }
        --indexOfOriginal;
    }
    //add a ‘\0‘ at last
    str[newLength-1] = ‘\0‘;
}

void test(char str[], int maxLength) {
    cout << "-----------test-------------" << endl;
    printf("Original: %s\n", str);
    ReplaceBlank(str, maxLength);
    printf("Replaced: %s\n", str);
    cout << "-----------end---------------" << endl;

}
int main() {
    /*
    char str[50] = "hello world and s";
    ReplaceBlank(str, 50);
    printf("%s\n", str);
     */
    char str1[50]="hello world and s";
    char str2[50]="";
    char str3[20]="hello world! fdsjkd";
    char str4[50]="    helloweroddfsd";
    char str5[50]="       ";
    test(str1, 50);
    test(str2, 50);
    test (NULL, 50);
    test(str3, 20);
    test(str4, 50);
    test(str5, 50);
    return 0;
}

参考资料：《剑指Offer名企面试官精讲典型编程题》