핸드폰 번호 가리기 - 1단계 (C++)

#include <string>
#include <vector>

using namespace std;

string solution(string phone_number) {
	string answer = "";
    for (int i = 0; i < phone_number.size(); i++){
    	if (i < phone_number.size() - 4) answer += "*";
        else answer += phone_number[i];
    }
    return answer;
}