[Level 1, C++] 핸드폰 번호 가리기

#include <string>
using namespace std;
string solution(string phone_number) {
    string answer = phone_number;
    for(int i=0; i<phone_number.length(); i++)
    {
        if(i < phone_number.length() - 4) answer[i] = '*';
        else answer[i] = phone_number[i];
    }
    return answer;
}