python 双指针

class Solution:

def EntryNodeOfLoop(self, pHead):
    # write code here
    if not pHead or not pHead.next:
        return 
    slow = pHead
    fast = pHead

    while fast.next.next:
        slow = slow.next
        fast = fast.next.next
        if slow == fast:
            break
    slow = pHead
    while slow != fast:
        slow = slow.next
        fast = fast.next
    return slow