Linked List Cycle

Description

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.

Constraints

The number of the nodes in the list is in the range [0, 10^4].
-10^5 <= Node.val <= 10^5
pos is -1 or a valid index in the linked-list.

Test Cases

Input: head = [3,2,0,-4], pos = 1
Output: true
Input: head = [1], pos = -1
Output: false

Code

Approach 1: `Using slow and fast pointers`

Explanation

Initialize two pointers slow and fast to head.
Move slow by one node and fast by two nodes.
If slow and fast meet, then there is a cycle.
Move fast to head and move both slow and fast by one node.
If slow and fast meet, then return slow.

Analysis

Time Complexity: O(n)
Space Complexity: O(1)

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