142.md

Linked List Cycle 2

Description

link

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Solution

• Floyd's Tortoise and Hare
• Cycle Detection

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Code

T : O(2m + n) S : O(1)

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

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        slow, fast = head, head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                slow2 = head
                while slow != slow2:
                    slow = slow.next
                    slow2 = slow2.next
                return slow
            
        return None