<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 TrieNode {
public:
  TrieNode(char in) {
    c = in;
    children = vector<TrieNode*>(26, nullptr);
  }

  char c;
  vector<TrieNode*> children;
  bool is_word = false;
  string word;
};

class Solution {
public:
    void insert(TrieNode* root, const string& word) {
      if (root == nullptr) return;
      TrieNode* curr_node = root;
      for (int i = 0; i < word.size(); ++i) {
        int j = word[i] - 'a';
        if (curr_node->children[j] == nullptr)
            curr_node->children[j] = new TrieNode(word[i]);
        curr_node = curr_node->children[j];
      }
      curr_node->word = word;
      curr_node->is_word = true;
    }

    void search(const TrieNode* curr_node, string& res) {
      if (curr_node == nullptr || !curr_node->is_word) return;
      if (curr_node->word.size() > res.size() ||
         (curr_node->word.size() == res.size() && curr_node->word < res)) {
        res = curr_node->word;
      }
      for (const TrieNode* child : curr_node->children) {
        if (child != nullptr) search(child, res);
      }
    }

    string longestWord(vector<string>& words) {
      TrieNode* root = new TrieNode(' ');
      for (const string& word : words) {
        insert(root, word);
      }
      string res;
      for (const TrieNode* child : root->children) {
        search(child, res);
      }
      return res;
    }
};
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