<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 {
public:
    class TrieNode {
    public:
        unordered_map <char, TrieNode*> children;
        bool finished;

        TrieNode() : finished(false) {}
    };

    TrieNode* root;
    
    void insert(string key) {
        TrieNode* node = root;

        for(int i = 0; i < key.length(); i++) {
            char ch = key[i];

            if (node->children[ch] == nullptr) {
                node->children[ch] = new TrieNode();
            }

            node = node->children[ch];   
        }  

        node->finished = true;
    }

    bool check(string word) {
        TrieNode* node = root;
        
        for(int i = 0; i < word.length(); i++) {
            char ch = word[i];
            TrieNode* child = node->children[ch];
            if (child == nullptr || !child->finished) {
                return false;
            }
            node = child;
        }
        return true;
    }

    string longestWord(vector<string>& words) {
        int n = words.size();
        root = new TrieNode();
        sort(words.begin(), words.end());

        for (int i = 0; i < n; ++i) {
            insert(words[i]);
        }

        string ans = "";
        for (int i = 0; i < n; ++i) {
            if (check(words[i]) && ans.size() < words[i].size()) {
                ans = words[i];
            }
        }

        return ans;
    }
};
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