<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 {

    struct Node {
        int cnt = -1;
        vector<Node*> child;
        Node() {
            child.resize(26);
        }
    } root;

    int max_cnt = 0;
    string res;

    void insert(const string &str) {
        Node *p = &root;
        int n = str.size(), val = -1;
        for (int i = 0; i < n; i += 1) {
            int idx = str[i] - 'a';
            if (!p->child[idx]) p->child[idx] = new Node();
            if (i == n - 1) {
                val = p->cnt;
            }
            p = p->child[idx];
        }
        if (val != -1) {
            p->cnt = val + 1;
        }
        if (p->cnt > max_cnt || p->cnt == max_cnt && str < res) {
            res = str;
            max_cnt = p->cnt;
        }
    }

public:
    string longestWord(vector<string>& words) {
        sort(words.begin(), words.end(), [](auto &a, auto &b) {
            return a.size() < b.size();
        });
        root.cnt = 0;
        for (auto &s: words) {
            insert(s);
        }
        return res;
    }
};
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