反转链表的几种方法

1-最基础

func reverseList_1(head *ListNode) *ListNode {
	if head == nil {
		return nil
	}
	if head.Next == nil {
		return head
	}
	p := head
	q := p.Next
	s := q.Next
	for s != nil {
		q.Next = p
		p = q
		q = s
		s = s.Next
	}
	q.Next = p
	head.Next = nil
	head = q
	return head
}

2-迭代

// 法2：迭代；
func reverseList_2(head *ListNode) *ListNode {
	var prev *ListNode
	cur := head
	for cur != nil {
		next := cur.Next
		cur.Next = prev
		prev = cur
		cur = next
	}
	return prev
}

3-递归

// 法3：递归；
func reverseList(head *ListNode) *ListNode {
	// 链表为空或只有单元素就不用再递归了；
	if head == nil || head.Next == nil {
		return head
	}
	p := reverseList(head.Next) // 递的过程；
	// 归的过程；
	head.Next.Next = head
	head.Next = nil
	return p
}