0720. Longest Word in Dictionary

https://leetcode.com/problems/longest-word-in-dictionary

Description

Given an array of strings words representing an English Dictionary, return 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".

Constraints:

• 1 <= words.length <= 1000

• 1 <= words[i].length <= 30

• words[i] consists of lowercase English letters.

ac1: trie

class Solution {
    public String longestWord(String[] words) {
        // edge cases
        if (words == null || words.length == 0) return "";

        TrieNode root = new TrieNode();
        for (String w : words) build(root, w);

        return search(root);
    }
    private void build(TrieNode root, String word) {
        for (int i = 0; i < word.length(); i++) {
            int pos = word.charAt(i) - 'a';
            if (root.next[pos] == null) root.next[pos] = new TrieNode();
            root = root.next[pos];
        }
        root.word = word;
    }
    private String search(TrieNode root) {
        String res = root.word != null ? root.word : "";
        for (TrieNode t : root.next) {
            if (t == null || t.word == null) continue; // null node or not a word in dict
            String child = search(t);
            if (child.length() > res.length()) res = child;
        }
        return res;
    }
}
class TrieNode {
    TrieNode[] next;
    String word;
    public TrieNode() {
        next = new TrieNode[26];
    }
}