문자열 치환하기(덮어쓰기)

using System;

public class Solution {
    public string solution(string my_string, string overwrite_string, int s) {
        string answer = "";

        answer = my_string.Substring(0,s);

        string end = my_string.Substring(s + overwrite_string.Length);

        answer = answer + overwrite_string + end;

        return answer;
    }
}