Find the K-Sum of an Array
<p>You are given an integer array <code>nums</code> and a <strong>positive</strong> integer <code>k</code>. You can choose any <strong>subsequence</strong> of the array and sum all of its elements together.</p>

<p>We define the <strong>K-Sum</strong> of the array as the <code>k<sup>th</sup></code> <strong>largest</strong> subsequence sum that can be obtained (<strong>not</strong> necessarily distinct).</p>

<p>Return <em>the K-Sum of the array</em>.</p>

<p>A <strong>subsequence</strong> is an array that can be derived from another array by deleting some or no elements without changing the order of the remaining elements.</p>

<p><strong>Note</strong> that the empty subsequence is considered to have a sum of <code>0</code>.</p>

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

<pre>
<strong>Input:</strong> nums = [2,4,-2], k = 5
<strong>Output:</strong> 2
<strong>Explanation:</strong> All the possible subsequence sums that we can obtain are the following sorted in decreasing order:
6, 4, 4, 2, <u>2</u>, 0, 0, -2.
The 5-Sum of the array is 2.
</pre>

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

<pre>
<strong>Input:</strong> nums = [1,-2,3,4,-10,12], k = 16
<strong>Output:</strong> 10
<strong>Explanation:</strong> The 16-Sum of the array is 10.
</pre>

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

<ul>
	<li><code>n == nums.length</code></li>
	<li><code>1 &lt;= n &lt;= 10<sup>5</sup></code></li>
	<li><code>-10<sup>9</sup> &lt;= nums[i] &lt;= 10<sup>9</sup></code></li>
	<li><code>1 &lt;= k &lt;= min(2000, 2<sup>n</sup>)</code></li>
</ul>
Array; Sorting; Heap (Priority Queue)
func kSum(nums []int, k int) int64 {
	mx := 0
	for i, x := range nums {
		if x > 0 {
			mx += x
		} else {
			nums[i] *= -1
		}
	}
	sort.Ints(nums)
	h := &hp{{0, 0}}
	for k > 1 {
		k--
		p := heap.Pop(h).(pair)
		if p.i < len(nums) {
			heap.Push(h, pair{p.sum + nums[p.i], p.i + 1})
			if p.i > 0 {
				heap.Push(h, pair{p.sum + nums[p.i] - nums[p.i-1], p.i + 1})
			}
		}
	}
	return int64(mx) - int64((*h)[0].sum)
}

type pair struct{ sum, i int }
type hp []pair

func (h hp) Len() int           { return len(h) }
func (h hp) Less(i, j int) bool { return h[i].sum < h[j].sum }
func (h hp) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
func (h *hp) Push(v any)        { *h = append(*h, v.(pair)) }
func (h *hp) Pop() any          { a := *h; v := a[len(a)-1]; *h = a[:len(a)-1]; return v }