LeetCode 合并K个升序链表

题目地址：https://leetcode.cn/problems/merge-k-sorted-lists/description/

算法思路：
先把每个链表的头加入小顶堆，把堆顶节点加入head并出堆，
如果这个节点的下一个元素不为空则加入堆并调整
直到所有元素全部出堆
具体实现如下

func mergeKLists(lists []*ListNode) *ListNode {
	size := 0
	heap := []*ListNode{}

	for i := 0; i < len(lists); i++ {
		if lists[i] != nil {
			size++
			heap = append(heap, lists[i])
		}
	}
	if size == 0 {
		return nil
	}
	for i := size/2 - 1; i >= 0; i-- {
		heapify(heap, i, size)
	}

	head := &ListNode{}
	tail := head
	for size > 0 {
		minNode := heap[0]
		tail.Next = minNode
		tail = tail.Next
		if minNode.Next != nil {
			heap[0] = minNode.Next
			heapify(heap, 0, size)
		} else {
			size--
			swap(heap, 0, size)
			heapify(heap, 0, size)
		}

	}
	return head.Next
}

// 调整小顶堆
func heapify(heap []*ListNode, i, size int) {
	left := i*2 + 1
	small := left
	for left < size {
		small = left
		if small+1 < size && heap[small].Val > heap[small+1].Val {
			small = left + 1
		}
		if heap[small].Val < heap[i].Val {
			swap(heap, small, i)
			i = small
			left = i*2 + 1
		} else {
			break
		}
	}
}

func swap(heap []*ListNode, i, j int) {
	heap[i], heap[j] = heap[j], heap[i]
}