【leetcode刷题】20T13-删除链表的倒数第N个节点

示例：
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.

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def removeNthFromEnd(self, head, n):
        """
        :type head: ListNode
        :type n: int
        :rtype: ListNode
        """
        # n有效，使得l2后移n个
        l1 = l2 = head
        for i in range(n):
            l2 = l2.next

        # 第一个节点
        if not l2:
            p = head
            head = head.next
            del(p)
            return head

        # l1和l2同时移动，l2到达末尾，则l1为倒数第n个
        while l2.next:
            l1 = l1.next
            l2 = l2.next

        # 删除l1.next
        p = l1.next
        l1.next = p.next
        del(p)
        return head