面试题05. 替换空格

输入：s = "We are happy."
输出："We%20are%20happy."

0 <= s 的长度 <= 10000

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