【剑指offer】面试题05：替换空格

05. 替换空格

请实现一个函数，把字符串 s 中的每个空格替换成"%20"。

输入：s = "We are happy."
输出："We%20are%20happy."

class Solution {
public:
    string replaceSpace(string s) {
        int len1 = s.length() - 1;
        for(int i = 0; i <= len1; i++){
            if(s[i] == ' ')
                s += "00";
        }
        int len2 = s.length() - 1;
        if(len2 <= len1)
            return s;
        for(int i = len1; i >= 0; i--){
            char c = s[i];
            if(c == ' '){
                s[len2--] = '0';
                s[len2--] = '2';
                s[len2--] = '%';
            }
            else
                s[len2--] = c;
        }
        return s;
    }
};