<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 Node {
    vector<Node*> children;
    int idx;
    bool ends = false;
public:
    Node () {
        vector<Node*> temp(26, NULL);
        children = temp;
        idx = -1;
    }

    bool isPresent(char ch) {
        return children[ch-'a'] != NULL;
    }

    void createChild(char ch) {
        children[ch-'a'] = new Node();
    }

    Node* getChildNode(char ch) {
        return children[ch-'a'];
    }

    void setFlag(int idx) {
        ends = true;
        this->idx = idx;
    }

    bool getFlag() {
        return ends;
    }

    int getIdx() {
        return idx;
    }
};

class Trie {
private:
    Node* root;
    vector<int> dirRow = {-1,0,1,0};
    vector<int> dirCol = {0,1,0,-1};

public:
    Trie() {
        root = new Node();
    }

    void insert(string word, int idx) {

        Node* node = root;

        for (int i=0; i<word.size(); i++) {
            char ch = word[i];

            if (!node->isPresent(ch)) {
                node->createChild(ch);
            }

            node = node->getChildNode(ch);
        }

        node->setFlag(idx);
    }

    void dfs (Node* node, int row, int col, vector<vector<char>>& board, int n, int m, vector<bool>& inBoard, vector<vector<bool>>& visited) {
        if (node->getFlag() && !inBoard[node->getIdx()]) {
            inBoard[node->getIdx()] = true;
        }

        visited[row][col] = true;

        for (int i=0; i<4; i++) {
            int newRow = row + dirRow[i];
            int newCol = col + dirCol[i];

            if (
                newRow >= 0 && newCol >= 0 &&
                newRow < n && newCol < m && 
                !visited[newRow][newCol] && 
                node->isPresent(board[newRow][newCol])
            ) 
            {
                char ch = board[newRow][newCol];

                dfs (node->getChildNode(ch), newRow, newCol, board, n, m, inBoard, visited);
            }
        }

        visited[row][col] = false;
    }

    void dfsOnEachCell(vector<vector<char>>& board, int n, int m, vector<bool>& inBoard) {
        for (int i=0; i<n; i++) {
            for (int j=0; j<m; j++) {
                if (root->isPresent(board[i][j])) {
                    vector<vector<bool>> visited(n, vector<bool>(m));
                    dfs(root->getChildNode(board[i][j]), i, j, board, n, m, inBoard, visited);
                }
            }
        }
    }
};

class Solution {
public:
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {

        int n = board.size();
        int m = board[0].size();

        vector<string> wordsOnBoard;

        Trie t;

        for (int i=0; i<words.size(); i++) {
            t.insert(words[i], i);
        }

        vector<bool> inBoard(words.size());

        t.dfsOnEachCell(board, n, m, inBoard);

        for (int i=0; i<words.size(); i++) {
            if (inBoard[i]) {
                wordsOnBoard.push_back(words[i]);
            }
        }

        return wordsOnBoard;
    }
};
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