148. Sort List #

Problem #

Sort a linked list in O(n log n) time using constant space complexity.

Example 1:


Input: 4->2->1->3
Output: 1->2->3->4

Example 2:


Input: -1->5->3->4->0
Output: -1->0->3->4->5

Summary #

Sort a linked list, requiring the time complexity to be O(n log n) and the space complexity to be O(1)

Solution Approach #

This problem can only use merge sort to meet the requirements. The 2 operations needed for merge sort have already appeared in other problems: finding the middle point is Problem 876, and merging 2 sorted linked lists is Problem 21.

Code #


package leetcode

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func sortList(head *ListNode) *ListNode {
	length := 0
	cur := head
	for cur != nil {
		length++
		cur = cur.Next
	}
	if length <= 1 {
		return head
	}

	middleNode := middleNode(head)
	cur = middleNode.Next
	middleNode.Next = nil
	middleNode = cur

	left := sortList(head)
	right := sortList(middleNode)
	return mergeTwoLists(left, right)
}

func middleNode(head *ListNode) *ListNode {
	if head == nil || head.Next == nil {
		return head
	}
	p1 := head
	p2 := head
	for p2.Next != nil && p2.Next.Next != nil {
		p1 = p1.Next
		p2 = p2.Next.Next
	}
	return p1
}

func mergeTwoLists(l1 *ListNode, l2 *ListNode) *ListNode {
	if l1 == nil {
		return l2
	}
	if l2 == nil {
		return l1
	}
	if l1.Val < l2.Val {
		l1.Next = mergeTwoLists(l1.Next, l2)
		return l1
	}
	l2.Next = mergeTwoLists(l1, l2.Next)
	return l2
}