# 141. Linked List Cycle

The strategy is to use two pointers, a walker and a runner (slow and fast). The walker (slow) moves forward one node at a time. The runner (fast) moves ahead 2 nodes at a time. So if there is a cycle, the runner will eventually lap the walker and the walker and runner will be at the same node. Otherwise the runner will reach the end of the linked list first.

# Definition for singly-linked list.

# class ListNode:

# def __init__(self, x):

# self.val = x

# self.next = None

class Solution:

def hasCycle(self, head: Optional[ListNode]) -> bool:

if not head or not head.next:

return False

walker = head

runner = head

while runner and runner.next:

walker = walker.next

runner = runner.next.next

if walker == runner:

return True

return False

Time Complexity: O(N).

Space complexity O(1)

Last modified 2yr ago