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Approach: Merge sort: split with slow/fast pointers, recursively sort halves, then merge two sorted lists.

Complexity: O(n log n) time, O(log n) stack space

go

/**
 * 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
	}
	slow, fast := head, head.Next
	for fast != nil && fast.Next != nil {
		slow = slow.Next
		fast = fast.Next.Next
	}
	mid := slow.Next
	slow.Next = nil
	return merge(sortList(head), sortList(mid))
}

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