297. Linked List Cycle

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.

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

Examples

// List: 3 → 2 → 0 → -4 → (back to 2)  →  true
// List: 1 → 2 → null                    →  false