Cyclic Chain Detection

Easy

You are given the starting node of a singly linked list. Your task is to determine whether the linked list contains a cycle — that is, whether any node in the chain eventually points back to a previously visited node, creating an infinite loop. A cycle exists in the linked list if following the next pointers from the head leads you back to a node you have already traversed. If such a cyclic path exists, return true . If the chain terminates naturally (i.e., reaches a null pointer), return false . Important: The pos parameter shown in the examples is used internally to construct the test case (indicating which node the tail connects to, forming a cycle). This parameter is not passed to your function — you must detect the cycle based solely on the structure of the linked list itself.

Example 1

Input

head = [3,2,0,-4], pos = 1

Output

true

Explanation

The linked list has 4 nodes with values [3, 2, 0, -4]. The tail node (-4) connects back to the node at index 1 (value 2), forming a cycle: 3 → 2 → 0 → -4 → 2 → 0 → -4 → ... (repeating infinitely). Since following the chain leads back to a previously visited node, a cycle exists.

Example 2

Input

head = [1,2], pos = 0

Output

true

Explanation

The linked list has 2 nodes with values [1, 2]. The tail node (2) connects back to the head node (value 1), forming a cycle: 1 → 2 → 1 → 2 → ... (repeating). A cyclic path is detected.

Example 3

Input

head = [1], pos = -1

Output

false

Explanation

The linked list contains only a single node with value 1, and it does not point to any other node (next is null). Since the chain terminates without revisiting any node, there is no cycle.

Constraints

The number of nodes in the list is in the range [0, 10⁴].
-10⁵ ≤ Node.val ≤ 10⁵
pos is -1 (indicating no cycle) or a valid index in the linked list (0-indexed) where the tail connects.
The pos parameter is for test case construction only and is not passed to your function.