Special Permutations
<p>You are given a&nbsp;<strong>0-indexed</strong>&nbsp;integer array&nbsp;<code>nums</code>&nbsp;containing&nbsp;<code>n</code>&nbsp;<strong>distinct</strong> positive integers. A permutation of&nbsp;<code>nums</code>&nbsp;is called special if:</p>

<ul>
	<li>For all indexes&nbsp;<code>0 &lt;= i &lt; n - 1</code>, either&nbsp;<code>nums[i] % nums[i+1] == 0</code>&nbsp;or&nbsp;<code>nums[i+1] % nums[i] == 0</code>.</li>
</ul>

<p>Return&nbsp;<em>the total number of special permutations.&nbsp;</em>As the answer could be large, return it&nbsp;<strong>modulo&nbsp;</strong><code>10<sup>9&nbsp;</sup>+ 7</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [2,3,6]
<strong>Output:</strong> 2
<strong>Explanation:</strong> [3,6,2] and [2,6,3] are the two special permutations of nums.
</pre>

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

<pre>
<strong>Input:</strong> nums = [1,4,3]
<strong>Output:</strong> 2
<strong>Explanation:</strong> [3,1,4] and [4,1,3] are the two special permutations of nums.
</pre>

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

<ul>
	<li><code>2 &lt;= nums.length &lt;= 14</code></li>
	<li><code>1 &lt;= nums[i] &lt;= 10<sup>9</sup></code></li>
</ul>
Bit Manipulation; Array; Dynamic Programming; Bitmask
func specialPerm(nums []int) (ans int) {
	const mod int = 1e9 + 7
	n := len(nums)
	m := 1 << n
	f := make([][]int, m)
	for i := range f {
		f[i] = make([]int, n)
	}
	for i := 1; i < m; i++ {
		for j, x := range nums {
			if i>>j&1 == 1 {
				ii := i ^ (1 << j)
				if ii == 0 {
					f[i][j] = 1
					continue
				}
				for k, y := range nums {
					if x%y == 0 || y%x == 0 {
						f[i][j] = (f[i][j] + f[ii][k]) % mod
					}
				}
			}
		}
	}
	for _, x := range f[m-1] {
		ans = (ans + x) % mod
	}
	return
}