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].

//Time: O(nlogn), Space: O(n) 
   public String longestWord(String[] words) {
        if (words == null || words.length == 0) {
            return null;
        }
        
        Arrays.sort(words);
        Set<String> set = new HashSet<String>();
        String result = "";
        
        for (int i = 0; i < words.length; i++) {
            String s = words[i];
            //千万不要忘记s.length() == 1的情况，因为开始set里面没有“”
            if (s.length() == 1 || set.contains(s.substring(0, s.length() - 1))) {
                //s.length()和result.length()比较的原因的结果要按照字母先后输出，比如apple和apply要输出的是apple
                result = s.length() > result.length() ? s : result;
                set.add(s);
            }
        }
        
        return result;
    }