class Solution {
public:
    string replaceSpace(string s) {
        size_t space = 0;
        for(auto ch:s)
        {
            if(ch == ' ')
                space++;
        }
        string ret;
        //避免增容消耗
        ret.reserve(s.size() + space*2);
        for(auto ch:s)
        {
            if(ch != ' ')
            {
                ret += ch;
            }
            else
            {
                ret += "%20";
            }
        }
        return ret;
    }
};