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].

Sort the words alphabetically, therefore shorter words always comes before longer words;
Along the sorted list, populate the words that can be built;
Any prefix of a word must come before that word.
注意Arrays.sort的时候，字母序优先于长度，更新res的时候要判断s的长度是否大于res的长度res = s.length() > res.length() ? s : res;

time: O(nlog(nL)) 　　-- L: average length of word, space: O(n)

class Solution {
    public String longestWord(String[] words) {
        Arrays.sort(words);
        
        String res = "";
        Set<String> set = new HashSet<>();
        for(String word : words) {
            if(word.length() == 1 || set.contains(word.substring(0, word.length() - 1))) {
                res = word.length() > res.length() ? word : res;
                set.add(word);
            }
        }
        return res;
    }
}