链表中环的入口节点

微信图片_20191222194317.jpg

class Solution:
    def EntryNodeOfLoop(self, pHead):
        # write code here
        if pHead is None or pHead.next is None:
            return None
        fast = pHead
        slow = pHead
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
            if fast == slow:
                break
        fast = pHead
        while fast != slow:
            fast = fast.next
            slow = slow.next
        return fast