<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 {
public:
    bool exist(vector<vector<char>>& board, string word) {
        size_t rows = board.size();
        size_t cols = board.front().size();
        std::reverse(word.begin(), word.end());
        char start = word.front();
        for (size_t i = 0; i < rows; ++i) {
            for (size_t j = 0; j < cols; ++j) {
                if (board[i][j] == start) {
                    std::set<std::pair<size_t, size_t>> used{};
                    if (check(board, rows, cols, i, j, word, 0, used)) {
                        return true;
                    }
                }
            }
        }
        return false;
    }
private:
    vector<std::pair<int, int>> dirs{ {-1, 0}, {0, -1}, {0, 1}, {1, 0} };

    bool check(
        vector<vector<char>>& board, size_t rows, size_t cols, size_t i, size_t j,
        string& word, size_t word_idx,
        std::set<std::pair<size_t, size_t>>& used
    ) {
        if (word_idx == word.length()-1) {
            return true;
        }
        std::pair<int, int> coords{ i, j };
        used.insert(coords);
        auto next_c = word[word_idx+1];
        for (auto [rd, cd] : dirs) {
            int x = i + rd;
            int y = j + cd;
            if (0 <= x && x < rows && 0 <= y && y < cols && !used.count({x, y})) {
                if (board[x][y] == next_c) {
                    // std::set<std::pair<size_t, size_t>> next_used = used;
                    if (check(board, rows, cols, x, y, word, word_idx+1, used)) {
                        return true;
                    }
                }
            }
        }
        used.erase(coords);
        return false;
    }
};
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