<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:
    struct TrieNode{
        TrieNode * next[256] = {0};
        bool is_leaf = false;
        string word;

        TrieNode() {;};
    };

    class Trie{
    public:
        TrieNode * root;

        Trie(){
            root = new TrieNode();
        };

        bool insert(const string & word){
            TrieNode * curr = root;
            for (auto a : word){
                if (curr->next[a] == nullptr){
                    curr->next[a] = new TrieNode();
                }
                curr = curr->next[a];
            }
            curr->is_leaf = true;
            curr->word = word;
            return true;
        }

        vector<string> search(const vector<vector<char>> & board){
            int m = board.size(), n = board[0].size(), dirs[] = {0, -1, 0, +1, 0};
            
            unordered_set<string> res;
            vector<vector<int>> visited(m+1, vector<int>(n + 1, 0));

            function<void(TrieNode *, int, int)> dfs =[&](TrieNode * node, int p, int q){
                if (node->is_leaf){
                    res.insert(node->word);
                }

                for (int d = 0; d < 4; ++d){
                    int x = p + dirs[d], y = q + dirs[d+1];
                    if (x < 0 || x >= m || y < 0 || y >= n || visited[x][y]){
                        continue;
                    }

                    char ch = board[x][y];
                    if (node->next[ch]){
                        visited[x][y] = true;
                        dfs(node->next[ch], x, y);
                        visited[x][y] = false;
                    }
                }
            };

            for (int i = 0; i < m; ++i){
                for (int j = 0; j < n; ++j){
                    if (root->next[board[i][j]]){
                        visited[i][j] = true;
                        dfs(root->next[board[i][j]], i, j);
                        visited[i][j] = false;
                    }
                }
            }
            return vector<string>(res.begin(), res.end());
        }
    };

    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        Trie trie;

        for (auto & word : words){
            trie.insert(word);
        }
        
        return trie.search(board);
    }
};
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