<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>

bool comp(string &a, string &b) {
    if (a.size() != b.size()) return a.size() < b.size() ;
    for (int i=0; i< a.size(); i++) {
        if (a[i] != b[i]) return a[i] < b[i] ;
    }
    return a < b; 
}

class Node {
    public : 
        char ch ; 
        int end ;
        Node * child[26] ;
        Node(char c, int en) {
            ch = c; 
            end =en ;
            for (int i=0; i< 26; i++) child[i] = NULL; 
        }
};

class Solution {
public:
    Node * root ;
    string ans = ""; 
    void create(string &s, Node * curr) {
        bool fl = false; 
        for (auto x : s) {
            int idx = x - 'a' ;
            if (curr->child[idx] == NULL) curr->child[idx] = new Node(x, 0) ;
            if (curr->end == 0) fl =true ;
            curr = curr->child[idx] ; 
        }
        curr->end = 1; 
        if (!fl && ans.size() < s.size()) ans = s ;
    }

    string longestWord(vector<string>& words) {
        sort(words.begin(), words.end(), comp) ;
        root = new Node ('/', 1) ;
        for (auto x : words) create(x , root) ; 
        return ans ; 
    }
};
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