19. 删除链表的倒数第N个节点

给定一个链表: 1->2->3->4->5, 和 n = 2.

当删除了倒数第二个节点后，链表变为 1->2->3->5.

type ListNode struct {
	Val  int
	Next *ListNode
}

func removeNthFromEnd(head *ListNode, n int) *ListNode {
	slow, fast := head, head
	for n != 0 {
		fast = fast.Next
		n--
	}
	if fast == nil {
		return head.Next
	}
	for fast.Next != nil {
		fast = fast.Next
		slow = slow.Next
	}
    // 最后一个节点指为空，然后返回head
	slow.Next = slow.Next.Next
	return head
}