链表中倒数最后k个结点

class Solution:
    def FindKthToTail(self , pHead: ListNode, k: int) -> ListNode:
        # write code here
        tail = pHead
        kth = pHead
        for _ in range(k):
            if kth:
                kth = kth.next
            else:
                return None
        while kth:
            tail = tail.next
            kth = kth.next
        return tail

class Solution:
    def FindKthToTail(self , pHead: ListNode, k: int) -> ListNode:
        # write code here
        slow = pHead
        fast = pHead
        index = 0
        while fast != None:
            fast = fast.next
            index += 1
            if index > k:
                slow = slow.next
        if index < k:
            return None
        else:
            return slow

class Solution:
    def FindKthToTail(self , pHead: ListNode, k: int) -> ListNode:
        # write code here
        l = []
        while pHead:
            l.append(pHead)
            pHead = pHead.next 
        if len(l) < k or k == 0:
            return None 
        return l[-k]

class Solution:
    def FindKthToTail(self , pHead: ListNode, k: int) -> ListNode:
        # write code here

        # 函数：求链表长度
        def lenth(pHead):
            n = 0
            while pHead:
                n += 1
                pHead = pHead.next
            return n

        # 判断长度是否小于k？
        l = lenth(pHead) 
        if l < k:
            return None
        else:
            n = l - k
            while n:
                pHead = pHead.next
                n -= 1
            return pHead

class Solution:
    def FindKthToTail(self , pHead: ListNode, k: int) -> ListNode:
        # write code here
        total_num = 0
        nh = ListNode(0)
        nh.next = pHead

        while pHead:
            pHead = pHead.next
            total_num += 1
        return_num = total_num - k
        if return_num<0:
            return None
        pHead = nh.next
        for i in range(return_num):
            pHead = pHead.next
        return pHead

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param pHead ListNode类 
# @param k int整型 
# @return ListNode类
#
class Solution:
    def FindKthToTail(self , pHead: ListNode, k: int) -> ListNode:
        # write code here
        slow =  quick = pHead

        while quick and k != 0:
            quick = quick.next
            k -= 1
        
        while quick:
            slow = slow.next
            quick = quick.next

        return slow if k == 0 else None

class Solution:
    def FindKthToTail(self , pHead: ListNode, k: int) -> ListNode:
        fast, slow = pHead, pHead
        for _ in range(k):
            if not fast: return None
            fast = fast.next
        while fast:
            fast = fast.next
            slow = slow.next
        return slow