전화번호 목록

import java.util.*;
class Solution {
    public boolean solution(String[] phone_book) {
        boolean answer = true;
        Map<String, Integer> hash = new HashMap<>();
        for (int i = 0; i < phone_book.length; i++) hash.put(phone_book[i], i);
        
        for (String number : phone_book) {
            for (int i = 1; i < number.length(); i++) {
                String prefix = number.substring(0, i);
                if (hash.containsKey(prefix)) answer = false;
            }
        }
        
        return answer;
    }
}