leetcode 19. 刪除鏈表的倒數第N個節點

給定一個鏈表，刪除鏈表的倒數第 n 個節點，而且返回鏈表的頭結點。

示例：

給定一個鏈表: 1->2->3->4->5, 和 n = 2.

當刪除了倒數第二個節點後，鏈表變爲 1->2->3->5.

說明：

給定的 n 保證是有效的。

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

class Solution:
    def removeNthFromEnd(self, head, n):
        """
        :type head: ListNode
        :type n: int
        :rtype: ListNode
        """
        tmp = head
        
        cnt = 0
        while tmp:
            cnt+=1
            tmp = tmp.next
        # print(cnt)

        ans = ListNode(-1)
        tmp = ans

        for _ in range(cnt-n):
            tmp.next = ListNode(head.val)
            tmp = tmp.next
            head = head.next

        head = head.next
        while head:
            tmp.next = ListNode(head.val)
            tmp = tmp.next
            head = head.next

        return ans.next