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시간 복잡도: O(len(n)), len(n) = length of phone_number

1#include <string>
2#include <vector>
3using namespace std;
4
5string solution(string phone_number) {
6  const int BOUND = phone_number.size()-4;
7  for(int i=0; i<BOUND; ++i) phone_number[i] = '*';
8  return phone_number;
9}