package q23

import "container/heap"

type ListNode struct {
	Val  int
	Next *ListNode
}

type MinNodeHp []*ListNode

func (m *MinNodeHp) Len() int               { return len(*m) }
func (m *MinNodeHp) Less(i int, j int) bool { return (*m)[i].Val < (*m)[j].Val }
func (m *MinNodeHp) Push(x any)             { *m = append(*m, x.(*ListNode)) }
func (m *MinNodeHp) Swap(i int, j int)      { (*m)[i], (*m)[j] = (*m)[j], (*m)[i] }

func (m *MinNodeHp) Pop() any {
	*m = (*m)[:len(*m)-1]
	return nil
}

var _ heap.Interface = &MinNodeHp{}

func mergeKLists(lists []*ListNode) *ListNode {
	hp := make(MinNodeHp, 0, len(lists))
	for i := range lists {
		if lists[i] != nil {
			hp = append(hp, lists[i])
		}
	}
	heap.Init(&hp)

	head := &ListNode{}
	tail := head

	for len(hp) > 0 {
		tail.Next = hp[0]
		tail = tail.Next

		hp[0] = hp[0].Next
		if hp[0] == nil {
			heap.Remove(&hp, 0)
		} else {
			heap.Fix(&hp, 0)
		}
	}

	tail.Next = nil
	return head.Next
}

var _ = mergeKLists