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
type Trie struct {
	children [26]*Trie
	ref      int
}

func newTrie() *Trie {
	return &Trie{ref: -1}
}
func (this *Trie) insert(w string, ref int) {
	node := this
	for _, c := range w {
		c -= 'a'
		if node.children[c] == nil {
			node.children[c] = newTrie()
		}
		node = node.children[c]
	}
	node.ref = ref
}

func findWords(board [][]byte, words []string) (ans []string) {
	trie := newTrie()
	for i, w := range words {
		trie.insert(w, i)
	}
	m, n := len(board), len(board[0])
	var dfs func(*Trie, int, int)
	dfs = func(node *Trie, i, j int) {
		idx := board[i][j] - 'a'
		if node.children[idx] == nil {
			return
		}
		node = node.children[idx]
		if node.ref != -1 {
			ans = append(ans, words[node.ref])
			node.ref = -1
		}
		c := board[i][j]
		board[i][j] = '#'
		dirs := [5]int{-1, 0, 1, 0, -1}
		for k := 0; k < 4; k++ {
			x, y := i+dirs[k], 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 i := 0; i < m; i++ {
		for j := 0; j < n; j++ {
			dfs(trie, i, j)
		}
	}
	return
}