LeetCode #141 Linked List Cycle

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Problem

Given a linked list, determine if it has a cycle in it.

Solution

Method: Two Pointer

Time Complexity: O(n)

Space Complexity: O(1)

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

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        slow = head
        fast = head
    
        while slow and fast.next and fast.next.next:
            fast = fast.next.next 
            if slow == fast:
                return True
            else:
                slow = slow.next
        
        return False

or

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

# This algorithm is more concise
class Solution(object):
    def hasCycle(self, head):
        slow = fast = head
        
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
        
            if slow == fast:
                return True

        return False

Pitfalls

  • the input linked list has no cycle => return false
  • the input linked list has cycle => return true