141. Linked List Cycle

Problem¶

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

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

Example 1:

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

Solve¶

Keep tracked of any node that we have pass by a hashmap. If we step in a tracked node, then there is a cycle, and we can return True.

Return False if otherwise

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        cache = set()
        p = head
        while p:
            if p in cache:
                return True
            cache.add(p)
            p = p.next

        return False