<p>Given an <code>m x n</code> <code>board</code>&nbsp;of characters and a list of strings <code>words</code>, return <em>all words on the board</em>.</p>

<p>Each word must be constructed from letters of sequentially adjacent cells, where <strong>adjacent cells</strong> are horizontally or vertically neighboring. The same letter cell may not be used more than once in a word.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/07/search1.jpg" style="width: 322px; height: 322px;" />
<pre>
<strong>Input:</strong> board = [[&quot;o&quot;,&quot;a&quot;,&quot;a&quot;,&quot;n&quot;],[&quot;e&quot;,&quot;t&quot;,&quot;a&quot;,&quot;e&quot;],[&quot;i&quot;,&quot;h&quot;,&quot;k&quot;,&quot;r&quot;],[&quot;i&quot;,&quot;f&quot;,&quot;l&quot;,&quot;v&quot;]], words = [&quot;oath&quot;,&quot;pea&quot;,&quot;eat&quot;,&quot;rain&quot;]
<strong>Output:</strong> [&quot;eat&quot;,&quot;oath&quot;]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/07/search2.jpg" style="width: 162px; height: 162px;" />
<pre>
<strong>Input:</strong> board = [[&quot;a&quot;,&quot;b&quot;],[&quot;c&quot;,&quot;d&quot;]], words = [&quot;abcb&quot;]
<strong>Output:</strong> []
</pre>

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

<ul>
	<li><code>m == board.length</code></li>
	<li><code>n == board[i].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 12</code></li>
	<li><code>board[i][j]</code> is a lowercase English letter.</li>
	<li><code>1 &lt;= words.length &lt;= 3 * 10<sup>4</sup></code></li>
	<li><code>1 &lt;= words[i].length &lt;= 10</code></li>
	<li><code>words[i]</code> consists of lowercase English letters.</li>
	<li>All the strings of <code>words</code> are unique.</li>
</ul>

class Solution {
public:
    vector<string> findWords(vector<vector<char>>& board,
                             vector<string>& words) {
        root_ = new Node();
        BuildTrie(words);
        int n = board.size();
        int m = board[0].size();
        unordered_set<string> ans;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                string w;
                vector<vector<bool>> seen(n, vector<bool>(m));
                FindWord(board, n, m, i, j, root_, w, seen, ans);
            }
        }

        return vector<string>(ans.begin(), ans.end());
    }

private:
    struct Node {
        vector<Node*> children;
        bool end = false;
        Node() { children = vector<Node*>(26, nullptr); }
    };

    void BuildTrie(const vector<string>& words) {
        for (const string& word : words) {
            AddWord(word);
        }
    }
    void AddWord(const string& word) {
        int n = word.size();
        auto cur = root_;
        for (int i = 0; i < n; i++) {
            if (!cur->children[word[i] - 'a']) {
                cur->children[word[i] - 'a'] = new Node();
            }
            cur = cur->children[word[i] - 'a'];
        }
        cur->end = true;
    }

    void FindWord(const vector<vector<char>>& board, int n, int m, int i, int j,
                  Node* root, string& w, vector<vector<bool>>& seen,
                  unordered_set<string>& ans) {
        if (seen[i][j]) return;
        char c = board[i][j];
        if (!root->children[c - 'a'])
            return;
        Node* cur = root->children[c - 'a'];
        w.push_back(c);
        seen[i][j] = true;
        if (cur->end) {
            ans.insert(w);
        }

        if (i + 1 < n)
            FindWord(board, n, m, i + 1, j, cur, w, seen, ans);
        if (i - 1 >= 0)
            FindWord(board, n, m, i - 1, j, cur, w, seen, ans);
        if (j - 1 >= 0)
            FindWord(board, n, m, i, j - 1, cur, w, seen, ans);
        if (j + 1 < m)
            FindWord(board, n, m, i, j + 1, cur, w, seen, ans);
        w.pop_back();
        seen[i][j] = false;
    }

    Node* root_;
};
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