替换空格

class Solution {
public:
    string replaceSpaces(string &str) {
        string s;
        for(int i = 0; i < str.size(); i++){
            if(str[i] == ' '){
                s += "%";
                s += "20";
            }else{
                s += str[i];
            }
        }
        return s;
    }
};