<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 {
private:
    int trie[300005][26];
    bool stop[300005];
    int node_count;

    void insert(const string& word){
        int node = 0;
        for(char c: word){
            if(trie[node][c-'a'] == 0) {
                trie[node][c-'a'] = ++node_count;
            }
            node = trie[node][c-'a'];
        }
        stop[node] = true;
    }

public:
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        for(auto const &word: words){
            insert(word);
        }
        set<string> res;
        for(int x=0; x<board.size(); x++){
            for(int y=0; y<board[0].size(); y++){
                dfs(board, x, y, 0, "", res);
            }
        }
        return vector<string>(res.begin(), res.end());
    }

    void dfs(vector<vector<char>>& board, int x, int y, int node, string word, set<string>& res){
        if(x<0||x>=board.size()||y<0||y>=board[0].size() || board[x][y] == ' ') return;

        if(trie[node][board[x][y]-'a'] != 0){
            word += board[x][y];
            node = trie[node][board[x][y]-'a'];
            if(stop[node]) res.insert(word);
            char c = board[x][y];
            board[x][y] = ' ';
            dfs(board, x+1, y, node, word, res);
            dfs(board, x-1, y, node, word, res);
            dfs(board, x, y+1, node, word, res);
            dfs(board, x, y-1, node, word, res);
            board[x][y] = c;
        }
    }
};
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