Description

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

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Follow up:

Can you solve it using O(1) (i.e. constant) memory?

Solutions

1. Two Pointers

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
# Time: O(n)
# Space: O(1)
class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        if not head or not head.next:
            return False
        faster, slower = head, head
        while faster and slower:
            if faster.next:
                faster = faster.next.next
            else:
                faster = None
            slower = slower.next
            if faster == slower:
                return True
        return False
# 17/17 cases passed (48 ms)
# Your runtime beats 89.92 % of python3 submissions
# Your memory usage beats 100 % of python3 submissions (16 MB)

References

  1. 141. Linked List Cycle