Sword finger offer 05 Replace spaces

Original link ： The finger of the sword Offer 05. Replace blank space

solution:

        One traversal is enough

class Solution {
public:
    string replaceSpace(string s) {
        string res;
        for(auto &x : s) {
            if(x == ' ') res += "%20";
            else res += x;
        }            
        return res;
    }
};