链表中倒数最后k个结点

# 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
        # 第一遍遍历记录下当前链表的长度
        # 节从1开始，节点位置：链表长度 - 倒数值 + 1
        lenList = 0
        cur = pHead
        while cur is not None:
            lenList = lenList + 1
            cur = cur.next
        if lenList < k :
            return ListNode(None).next
        
        objNodeIdx = lenList - k + 1
        cur = pHead
        for i in range(1,objNodeIdx):# 位置从1开始计数
            cur = cur.next
        return cur

from typing import Optional

class Solution:
    def FindKthToTail(self , pHead: ListNode, k: int) -> Optional[ListNode]:
        a = b = pHead
        while a:
            a = a.next
            if not (k := k - 1):
                while a:
                    a, b = a.next, b.next
                return b

from math import e
# 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, k: int):
        # write code here
        if not pHead&nbs***bsp;k < 1:
            return None
        lastNode = pHead

        for _ in range(k-1):
            if lastNode.next:
                lastNode = lastNode.next
            else:
                return None
        while lastNode.next:
            lastNode = lastNode.next
            pHead = pHead.next
        return pHead

class Solution:
    def FindKthToTail(self , pHead: ListNode, k: int) -> ListNode:
        # write code here
        p, count, len1 = pHead, 0, self.Lenth(pHead)
        q = ListNode(0)
        if len1 < k:
            return q.next   # 返回空链表
        while True:
            if count == len1 - k:   # 遍历到倒数第k节点，输出
                return p
            p = p.next
            count = count + 1

        
    def Lenth(self, pHead): # 计算链表长度
        count  = 0
        while pHead:
            pHead = pHead.next
            count = count + 1
        return count

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