148.sort-list.go

/*
 * @lc app=leetcode id=148 lang=golang
 *
 * [148] Sort List
 */
/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
//   (8 ms) √ Your runtime beats 99.23 %
// merge sort
func sortList(head *ListNode) *ListNode {
	if head == nil || head.Next == nil {
		return head
	}

	slow, fast := head, head.Next
	for fast.Next != nil && fast.Next.Next != nil {
		slow = slow.Next
		fast = fast.Next.Next
	}
	// 右边可能多一个节点
	slow, slow.Next = slow.Next, nil // 注意，一定要断开两个链表
	// mid节点是slow
	l := sortList(head)
	r := sortList(slow)
	return merge(l, r)
}

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