Number of Subarrays With GCD Equal to K
<p>Given an integer array <code>nums</code> and an integer <code>k</code>, return <em>the number of <strong>subarrays</strong> of </em><code>nums</code><em> where the greatest common divisor of the subarray&#39;s elements is </em><code>k</code>.</p>

<p>A <strong>subarray</strong> is a contiguous non-empty sequence of elements within an array.</p>

<p>The <strong>greatest common divisor of an array</strong> is the largest integer that evenly divides all the array elements.</p>

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

<pre>
<strong>Input:</strong> nums = [9,3,1,2,6,3], k = 3
<strong>Output:</strong> 4
<strong>Explanation:</strong> The subarrays of nums where 3 is the greatest common divisor of all the subarray&#39;s elements are:
- [9,<u><strong>3</strong></u>,1,2,6,3]
- [9,3,1,2,6,<u><strong>3</strong></u>]
- [<u><strong>9,3</strong></u>,1,2,6,3]
- [9,3,1,2,<u><strong>6,3</strong></u>]
</pre>

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

<pre>
<strong>Input:</strong> nums = [4], k = 7
<strong>Output:</strong> 0
<strong>Explanation:</strong> There are no subarrays of nums where 7 is the greatest common divisor of all the subarray&#39;s elements.
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 1000</code></li>
	<li><code>1 &lt;= nums[i], k &lt;= 10<sup>9</sup></code></li>
</ul>
Array; Math; Number Theory
func subarrayGCD(nums []int, k int) (ans int) {
	for i := range nums {
		g := 0
		for _, x := range nums[i:] {
			g = gcd(g, x)
			if g == k {
				ans++
			}
		}
	}
	return
}

func gcd(a, b int) int {
	if b == 0 {
		return a
	}
	return gcd(b, a%b)
}