[String]720. Longest Word in Dic

题目：720. Longest Word in Dictionary

Given a list of strings words representing an English Dictionary, find the longest word in words that can be built one character at a time by other words in words. If there is more than one possible answer, return the longest word with the smallest lexicographical order.

If there is no answer, return the empty string.
Example 1:

Input: 
words = ["w","wo","wor","worl", "world"]
Output: "world"
Explanation: 
The word "world" can be built one character at a time by "w", "wo", "wor", and "worl".

Example 2:

Input: 
words = ["a", "banana", "app", "appl", "ap", "apply", "apple"]
Output: "apple"

Explanation:
Both "apply" and "apple" can be built from other words in the dictionary. However, "apple" is lexicographically smaller than "apply".
Note:

All the strings in the input will only contain lowercase letters.
The length of words will be in the range [1, 1000].
The length of words[i] will be in the range [1, 30].

Solution:
先排序！＋HashSet
Runtime: 35 ms

class Solution {
    public String longestWord(String[] words) {
        Arrays.sort(words);//sort first, so don't need to consider the lexicographically order
        HashSet set = new HashSet();
        String output = "";
        for(String word:words){
            if(word.length() == 1 || set.contains(word.substring(0, word.length()-1))){
                set.add(word);
                if(word.length() > output.length()){
                    output = word;
                }
            }
        }
        return output;
        
    }
}