<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 TrieNode {
public:
    vector<TrieNode*> child;
    bool isEnd;
    TrieNode() {
        for (int i = 0; i < 26; i++) {
            child.push_back(NULL);
        }
        isEnd = false;
    }
};

class Solution {
    TrieNode *root;
    set<string> ans;
    vector<pair<int, int>> nbr = {
        {0, -1},
        {0, 1},
        {-1, 0},
        {1, 0}
    };
    int nRow = 0;
    int nCol = 0;
public:
    Solution() {
        root = new TrieNode();
    }

    void insert(string word) {
        TrieNode *curr = root;
        for (auto it: word) {
            if (curr->child[it - 'a'] == NULL) {
                curr->child[it - 'a'] = new TrieNode();
            }
            curr = curr->child[it - 'a'];
        }
        curr->isEnd = true;
    }

    bool isValidNbr(int row, int col) {
        return row >= 0 && row < nRow && col >= 0 && col < nCol;
    }

    bool dfs(vector<vector<char>>& board, string &word, int wordIdx, int row, int col) {
        if (wordIdx == word.size() - 1 && board[row][col] == word[wordIdx]) {
            return true;
        }
        if (board[row][col] != word[wordIdx] && board[row][col] != '$') {
            return false;
        }

        char tmp = board[row][col];
        board[row][col] = '$';

        for (auto it: nbr) {
            if (isValidNbr(row + it.first, col + it.second) && board[row + it.first][col + it.second] != '$' && dfs(board, word, wordIdx + 1, row + it.first, col + it.second)) {
                board[row][col] = tmp;
                return true;
            }
        }
        board[row][col] = tmp;
        return false;
    }

    void modifiedDFS(vector<vector<char>>& board, string word, int row, int col, TrieNode *curr) {
        char ch = board[row][col];
        word = word + ch;
        if (curr->child[ch - 'a'] == NULL) {
            return;
        }
        board[row][col] = '$';

        if(curr->child[ch - 'a']->isEnd == true) {
            ans.insert(word);
        }

        for (auto it: nbr) {
            if (isValidNbr(row + it.first, col + it.second) && board[row + it.first][col + it.second] != '$') {
                modifiedDFS(board, word, row + it.first, col + it.second, curr->child[ch - 'a']);
            }
        }

        board[row][col] = ch;
    }

    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        nRow = board.size();
        nCol = board[0].size();
        // set<string> ans;
        // for (auto it: words) {
        //     for (int i = 0; i < nRow; i++) {
        //         for (int j = 0; j < nCol; j++) {
        //             if (ans.find(it) == ans.end() && board[i][j] == it[0] && dfs(board, it, 0, i, j)) {
        //                 ans.insert(it);
        //             }
        //         }
        //     }
        // }
        // return vector<string>(ans.begin(), ans.end());

        for (auto it: words) {
            insert(it);
        }

        for (int i = 0; i < nRow; i++) {
            for (int j = 0; j < nCol; j++) {
                modifiedDFS(board, "", i, j, root);
            }
        }
        return vector<string>(ans.begin(), ans.end());
    }
};
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