[Level 1][C++] 핸드폰 번호 가리기

#include <string>
#include <vector>

using namespace std;

string solution(string phone_number) {
    string answer = "";
    int idx=0;
    for(idx=0; idx<phone_number.size()-4; idx++)
        answer += "*"; 
    
    for(int i=idx; i<phone_number.size(); i++)
        answer += phone_number.at(i);       
  
    return answer;
}