Count the Number of K-Free Subsets
<p>You are given an integer array <code>nums</code>,&nbsp;which contains <strong>distinct</strong> elements and an integer <code>k</code>.</p>

<p>A subset is called a <strong>k-Free</strong> subset if it contains <strong>no</strong> two elements with an absolute difference equal to <code>k</code>. Notice that the empty set is a <strong>k-Free</strong> subset.</p>

<p>Return <em>the number of <strong>k-Free</strong> subsets of </em><code>nums</code>.</p>

<p>A <b>subset</b> of an array is a selection of elements (possibly none) of the array.</p>

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

<pre>
<strong>Input:</strong> nums = [5,4,6], k = 1
<strong>Output:</strong> 5
<strong>Explanation:</strong> There are 5 valid subsets: {}, {5}, {4}, {6} and {4, 6}.
</pre>

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

<pre>
<strong>Input:</strong> nums = [2,3,5,8], k = 5
<strong>Output:</strong> 12
<strong>Explanation:</strong> There are 12 valid subsets: {}, {2}, {3}, {5}, {8}, {2, 3}, {2, 3, 5}, {2, 5}, {2, 5, 8}, {2, 8}, {3, 5} and {5, 8}.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> nums = [10,5,9,11], k = 20
<strong>Output:</strong> 16
<strong>Explanation:</strong> All subsets are valid. Since the total count of subsets is 2<sup>4 </sup>= 16, so the answer is 16. 
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 50</code></li>
	<li><code>1 &lt;= nums[i] &lt;= 1000</code></li>
	<li><code>1 &lt;= k &lt;= 1000</code></li>
</ul>
Array; Math; Dynamic Programming; Combinatorics; Sorting
func countTheNumOfKFreeSubsets(nums []int, k int) int64 {
	sort.Ints(nums)
	g := map[int][]int{}
	for _, x := range nums {
		g[x%k] = append(g[x%k], x)
	}
	ans := int64(1)
	for _, arr := range g {
		m := len(arr)
		f := make([]int64, m+1)
		f[0] = 1
		f[1] = 2
		for i := 2; i <= m; i++ {
			if arr[i-1]-arr[i-2] == k {
				f[i] = f[i-1] + f[i-2]
			} else {
				f[i] = f[i-1] * 2
			}
		}
		ans *= f[m]
	}
	return ans
}