<p>Given an array of strings <code>words</code> representing an English Dictionary, return <em>the longest word in</em> <code>words</code> <em>that can be built one character at a time by other words in</em> <code>words</code>.</p>

<p>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.</p>

<p>Note that the word should be built from left to right with each additional character being added to the end of a previous word.&nbsp;</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> words = [&quot;w&quot;,&quot;wo&quot;,&quot;wor&quot;,&quot;worl&quot;,&quot;world&quot;]
<strong>Output:</strong> &quot;world&quot;
<strong>Explanation:</strong> The word &quot;world&quot; can be built one character at a time by &quot;w&quot;, &quot;wo&quot;, &quot;wor&quot;, and &quot;worl&quot;.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> words = [&quot;a&quot;,&quot;banana&quot;,&quot;app&quot;,&quot;appl&quot;,&quot;ap&quot;,&quot;apply&quot;,&quot;apple&quot;]
<strong>Output:</strong> &quot;apple&quot;
<strong>Explanation:</strong> Both &quot;apply&quot; and &quot;apple&quot; can be built from other words in the dictionary. However, &quot;apple&quot; is lexicographically smaller than &quot;apply&quot;.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= words.length &lt;= 1000</code></li>
	<li><code>1 &lt;= words[i].length &lt;= 30</code></li>
	<li><code>words[i]</code> consists of lowercase English letters.</li>
</ul>

class Solution {
    class TrieNode {
        public:
        vector<TrieNode*> next;
        bool isWord;
        string theWord;
        //public:
        TrieNode() {
            next.resize(26);
            isWord = false;
            theWord = "";
        }
    };
    class Trie {
        public:
        TrieNode* root;
        //public:
        Trie() {
            root = new TrieNode();
        }
        void insert(string word) {
            TrieNode* curr = root;
            for(int i = 0; i < word.size(); i++) {
                if(curr -> next[word[i] - 'a'] == NULL) {
                    curr -> next[word[i] - 'a'] = new TrieNode();
                }
                curr = curr -> next[word[i] - 'a'];
                if(i == word.size()-1) {
                    curr -> isWord = true;
                    curr -> theWord = word;
                }
            }
        }
    };
    void longest(string &ans, int &maxlen, int count, TrieNode* root) {
        for(int i = 0; i < 26; i++) {
            if(root -> next[i] != NULL) {
                TrieNode* nextNode = root -> next[i];
                if(nextNode -> isWord == true) {
                    if(count + 1 > maxlen) {
                        ans = nextNode -> theWord;
                        maxlen = count + 1;
                    }
                    //else {
                        longest(ans, maxlen, count+1, nextNode);
                    //}
                }
            }
        }
    }
public:
    string longestWord(vector<string>& words) {
        Trie trie;
        for(string word : words) {
            trie.insert(word);
        }
        string ans;
        int maxlen = 0;
        longest(ans, maxlen, 0, trie.root);
        return ans;
    }
};
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