剑指 Offer II 022. 链表中环的入口节点

剑指 Offer II 022. 链表中环的入口节点

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


class Solution:
    def detectCycle(self, head: ListNode) -> ListNode:
        #首先判断是不是有环
        if not head:
            return 
        elif head.next is None:
            return None
        elif head.next.next is None:
            return None

        last, end = head, head

        flag = False
        while last and end:

            last = last.next
            if end.next: 
                end = end.next.next
            else:
                return None
            if last == end:
                flag = True 
                break
        #确定环的输入口
        if flag:
            end = head
            while 1:
                if last == end:
                    return last
                last = last.next
                end = end.next

        else:
            return None