class ListNode:
    def __init__(self,x):
        self.val = x
        self.next = None
class Solution:
    def FindKthToTail(self,head,k):
        if not head or not k:
            return None
        left,right = head,head
        for i in range(k-1):
            if not right.next:
                return None
            right = right.next
        while right.next:
            left = left.next
            right = right.next
        return left