링크 : https://school.programmers.co.kr/learn/courses/30/lessons/12918?language=cpp
#include <string>
#include <iostream>
using namespace std;
bool solution(string s) {
bool answer = true;
if ((s.length() != 4 && s.length() != 6))
return false;
for(int i = 0; i < s.length(); i++){
if(isdigit(s[i]) == false)
answer = false;
}
return answer;
}