N-Queens
<p>The <strong>n-queens</strong> puzzle is the problem of placing <code>n</code> queens on an <code>n x n</code> chessboard such that no two queens attack each other.</p>

<p>Given an integer <code>n</code>, return <em>all distinct solutions to the <strong>n-queens puzzle</strong></em>. You may return the answer in <strong>any order</strong>.</p>

<p>Each solution contains a distinct board configuration of the n-queens&#39; placement, where <code>&#39;Q&#39;</code> and <code>&#39;.&#39;</code> both indicate a queen and an empty space, respectively.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0051.N-Queens/images/queens.jpg" style="width: 600px; height: 268px;" />
<pre>
<strong>Input:</strong> n = 4
<strong>Output:</strong> [[&quot;.Q..&quot;,&quot;...Q&quot;,&quot;Q...&quot;,&quot;..Q.&quot;],[&quot;..Q.&quot;,&quot;Q...&quot;,&quot;...Q&quot;,&quot;.Q..&quot;]]
<strong>Explanation:</strong> There exist two distinct solutions to the 4-queens puzzle as shown above
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 1
<strong>Output:</strong> [[&quot;Q&quot;]]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 9</code></li>
</ul>
Array; Backtracking
func solveNQueens(n int) (ans [][]string) {
	col := make([]int, n)
	dg := make([]int, n<<1)
	udg := make([]int, n<<1)
	t := make([][]byte, n)
	for i := range t {
		t[i] = make([]byte, n)
		for j := range t[i] {
			t[i][j] = '.'
		}
	}
	var dfs func(int)
	dfs = func(i int) {
		if i == n {
			tmp := make([]string, n)
			for i := range tmp {
				tmp[i] = string(t[i])
			}
			ans = append(ans, tmp)
			return
		}
		for j := 0; j < n; j++ {
			if col[j]+dg[i+j]+udg[n-i+j] == 0 {
				col[j], dg[i+j], udg[n-i+j] = 1, 1, 1
				t[i][j] = 'Q'
				dfs(i + 1)
				t[i][j] = '.'
				col[j], dg[i+j], udg[n-i+j] = 0, 0, 0
			}
		}
	}
	dfs(0)
	return
}