package main

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func sortList(head *ListNode) *ListNode {
	if head == nil || head.Next == nil {
		return head
	}
	// !!Must split []int{1, 2} into 1 & 2
	slow, fast := head, head.Next // So, fast = head.Next
	for fast != nil && fast.Next != nil {
		slow = slow.Next
		fast = fast.Next.Next
	}
	left := head
	right := slow.Next
	slow.Next = nil // Unlink in the middle
	left = sortList(left)
	right = sortList(right)
	return mergeList(left, right)
}

func mergeList(l1, l2 *ListNode) *ListNode {
	dummy := &ListNode{}
	prev := dummy
	for l1 != nil && l2 != nil {
		if l1.Val < l2.Val {
			prev.Next = l1
			l1 = l1.Next
		} else {
			prev.Next = l2
			l2 = l2.Next
		}
		prev = prev.Next
	}
	if l1 != nil {
		prev.Next = l1
	} else {
		prev.Next = l2
	}
	return dummy.Next
}

// func main() {
// 	l1 := toLinkedList([]int{
// 		2, 7, 4, 2, 8, 54, 3, 2})
// 	printList(sortList(l1))
// }