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快慢指针参考

# Definition for singly-linked list.class ListNode(object):    def __init__(self, x):        self.val = x        self.next = Noneclass Solution(object):    def detectCycle(self, head):        """        :type head: ListNode        :rtype: ListNode        """        slow, fast = head, head        flag = False        while fast and fast.next:            slow = slow.next            fast = fast.next.next            if slow == fast:                flag = True                break        if not flag:            return None        pos = head        while pos != slow:            pos = pos.next            slow = slow.next        return pos

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