<p>Given an <code>m x n</code> grid of characters <code>board</code> and a string <code>word</code>, return <code>true</code> <em>if</em> <code>word</code> <em>exists in the grid</em>.</p>

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

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/04/word2.jpg" style="width: 322px; height: 242px;" />
<pre>
<strong>Input:</strong> board = [[&quot;A&quot;,&quot;B&quot;,&quot;C&quot;,&quot;E&quot;],[&quot;S&quot;,&quot;F&quot;,&quot;C&quot;,&quot;S&quot;],[&quot;A&quot;,&quot;D&quot;,&quot;E&quot;,&quot;E&quot;]], word = &quot;ABCCED&quot;
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/04/word-1.jpg" style="width: 322px; height: 242px;" />
<pre>
<strong>Input:</strong> board = [[&quot;A&quot;,&quot;B&quot;,&quot;C&quot;,&quot;E&quot;],[&quot;S&quot;,&quot;F&quot;,&quot;C&quot;,&quot;S&quot;],[&quot;A&quot;,&quot;D&quot;,&quot;E&quot;,&quot;E&quot;]], word = &quot;SEE&quot;
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/10/15/word3.jpg" style="width: 322px; height: 242px;" />
<pre>
<strong>Input:</strong> board = [[&quot;A&quot;,&quot;B&quot;,&quot;C&quot;,&quot;E&quot;],[&quot;S&quot;,&quot;F&quot;,&quot;C&quot;,&quot;S&quot;],[&quot;A&quot;,&quot;D&quot;,&quot;E&quot;,&quot;E&quot;]], word = &quot;ABCB&quot;
<strong>Output:</strong> false
</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;= 6</code></li>
	<li><code>1 &lt;= word.length &lt;= 15</code></li>
	<li><code>board</code> and <code>word</code> consists of only lowercase and uppercase English letters.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Follow up:</strong> Could you use search pruning to make your solution faster with a larger <code>board</code>?</p>

class Solution {
private:
    vector<vector<bool>> visited;

public:
    bool exist(vector<vector<char>>& board, string word) {
        if (board.empty())
            return false;
        int m = board.size(), n = board[0].size();

        visited = vector<vector<bool>>(m, vector<bool>(n, false));
        if (word.size() > m * n)
            return false;

        for (char e : word) {
            bool exist = false;
            for (vector<char> vc : board)
                for (char c : vc)
                    if(c==e){
                        exist=true;
                        break;
                    }
            if(exist==false) return false;
        }
        if (m == 1 && n == 1 && word.size() == 1 && board[0][0] == word[0])
            return true;
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++) {
                if (backtracking(board, word, m, n, i, j, 0)) {
                    return true;
                }
            }
        return false;
    }
    vector<pair<int, int>> getNext(int row, int col, int m, int n) {
        vector<pair<int, int>> res;
        int top, up, left, right;
        left = col - 1;
        right = col + 1;
        top = row - 1;
        up = row + 1;
        if (top >= 0)
            res.push_back({top, col});
        if (right < n)
            res.push_back({row, right});
        if (left >= 0)
            res.push_back({row, left});
        if (up < m)
            res.push_back({up, col});
        return res;
    }
    bool backtracking(vector<vector<char>>& board, string word, int m, int n,
                      int row, int col, int index) {
        // cout<<board[row][col]<<index<<endl;
        if (index == word.size()) {
            // cout<<"find:"<<path<<endl;
            return true;
        } else if (index > word.size()) {
            return false;
        }
        if (word[index] == board[row][col]) {
            vector<pair<int, int>> next_E = getNext(row, col, m, n);
            visited[row][col] = true;
            if (index == word.size() - 1)
                return true;
            for (const auto [i, j] : next_E) {
                if (visited[i][j])
                    continue;                
                if (backtracking(board, word, m, n, i, j, index + 1))
                    return true;
            }
            visited[row][col] = false;
        }
        return false;
    }
};
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