大神写法

class Solution {
public:
    string replaceSpaces(string &str) {
        size_t t;
        while((t=str.find(' '))!=string::npos){
            str.replace(t,1,"%20");
        }
        return str;
    }
};

算法 1

class Solution {
public:
    string replaceSpaces(string &str) {
        string x;
        for(auto c : str) {
            if(c == ' ') x += "%20";
            else x += c;
        }
        return x;
    }
};

算法 2

class Solution {
public:
    string replaceSpaces(string &str) {
        int len = 0;
        for(auto c : str) 
            if(c == ' ') len += 3;
            else len ++;
        int i = str.size() - 1 , j = len - 1;
        str.resize(len);
        while(i >= 0) {
            if(str[i] == ' ') {
                str[j -- ] = '0';
                str[j -- ] = '2';
                str[j -- ] = '%';
            }
            else str[j -- ] = str[i];
            i --;
        }
        return str;
    }
};