<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:
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        using u6 = uint64_t;
        const u6 K = 27;
        
        unordered_set<u6> hs;

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

        vector<char> visited(m*n);
        int n_visited = 0;
        u6 h = 0;
        function<void(int, int)> walk = [&](int i, int j) {
            if (i < 0 || i >= m || j < 0 || j >= n) return;
            if (visited[i*n+j]) return;
            visited[i*n+j] = true;
            n_visited++;
            u6 c = board[i][j] - 'a' + 1;
            h = K*h+c;
            hs.insert(h);
            if (n_visited < 10) {
                for (auto [di, dj] : initializer_list<pair<int, int>>{
                    {1, 0}, {-1, 0}, {0, 1}, {0, -1}
                }) {
                    walk(i+di, j+dj);
                }
            }
            h /= K;
            visited[i*n+j] = false;
            n_visited--;
        };

        for (int i0 = 0; i0 < m; i0++) {
            for (int j0 = 0; j0 < n; j0++) {
                walk(i0, j0);
                assert(h == 0);
            }
        }

        erase_if(words, [&hs](auto const& w) {
            u6 h = 0;
            for (char c : w) {
                h = K*h+c-'a'+1;
            }
            return !hs.contains(h);
        });

        return words;
    }
};
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