强行用c++写

class Solution {
public:
    void replaceSpace(char *str,int length) {
        string s(str);
       string res;
        for(auto &x:s){
            if(x==' ')
                res+="%20";
            else
                res+=x;
        }
        auto ret=res.c_str();
        strcpy(str,ret);
    }
};