N-Queens II
<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>the number of distinct solutions to the&nbsp;<strong>n-queens puzzle</strong></em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0052.N-Queens%20II/images/queens.jpg" style="width: 600px; height: 268px;" />
<pre>
<strong>Input:</strong> n = 4
<strong>Output:</strong> 2
<strong>Explanation:</strong> There are two distinct solutions to the 4-queens puzzle as shown.
</pre>

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

<pre>
<strong>Input:</strong> n = 1
<strong>Output:</strong> 1
</pre>

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

<ul>
	<li><code>1 &lt;= n &lt;= 9</code></li>
</ul>
Backtracking
func totalNQueens(n int) (ans int) {
	cols := [10]bool{}
	dg := [20]bool{}
	udg := [20]bool{}
	var dfs func(int)
	dfs = func(i int) {
		if i == n {
			ans++
			return
		}
		for j := 0; j < n; j++ {
			a, b := i+j, i-j+n
			if cols[j] || dg[a] || udg[b] {
				continue
			}
			cols[j], dg[a], udg[b] = true, true, true
			dfs(i + 1)
			cols[j], dg[a], udg[b] = false, false, false
		}
	}
	dfs(0)
	return
}