Word Search II
<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://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0200-0299/0212.Word%20Search%20II/images/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://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0200-0299/0212.Word%20Search%20II/images/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>
Trie; Array; String; Backtracking; Matrix
class Trie {
    children: Trie[];
    ref: number;

    constructor() {
        this.children = new Array(26);
        this.ref = -1;
    }

    insert(w: string, ref: number): void {
        let node: Trie = this;
        for (let i = 0; i < w.length; i++) {
            const c = w.charCodeAt(i) - 97;
            if (node.children[c] == null) {
                node.children[c] = new Trie();
            }
            node = node.children[c];
        }
        node.ref = ref;
    }
}

function findWords(board: string[][], words: string[]): string[] {
    const tree = new Trie();
    for (let i = 0; i < words.length; ++i) {
        tree.insert(words[i], i);
    }
    const m = board.length;
    const n = board[0].length;
    const ans: string[] = [];
    const dirs: number[] = [-1, 0, 1, 0, -1];
    const dfs = (node: Trie, i: number, j: number) => {
        const idx = board[i][j].charCodeAt(0) - 97;
        if (node.children[idx] == null) {
            return;
        }
        node = node.children[idx];
        if (node.ref != -1) {
            ans.push(words[node.ref]);
            node.ref = -1;
        }
        const c = board[i][j];
        board[i][j] = '#';
        for (let k = 0; k < 4; ++k) {
            const x = i + dirs[k];
            const y = j + dirs[k + 1];
            if (x >= 0 && x < m && y >= 0 && y < n && board[x][y] != '#') {
                dfs(node, x, y);
            }
        }
        board[i][j] = c;
    };
    for (let i = 0; i < m; ++i) {
        for (let j = 0; j < n; ++j) {
            dfs(tree, i, j);
        }
    }
    return ans;
}
