[프로그래머스] 전화번호 목록 - Hash (Java)

import java.util.*;

class Solution {
	public boolean solution(String[] phone_book) {
		boolean answer = true;

		HashMap<String, Integer> hm = new HashMap<>();

		for (String num : phone_book) {
			hm.put(num, 0);
		}

		for (String num : hm.keySet()) {
			for (String key : hm.keySet()) {
				if (!key.equals(num) && key.startsWith(num)) {
					answer = false;
					break;
				}
			}
		}
		return answer;
	}
}