by Pawan Kumar @jsartisan
Write a function that takes the head node of a linked list and checks if the list contains a cycle. Return true if a cycle exists, and false if not.
A cycle occurs when following the next pointers eventually leads back to a previously visited node, creating a loop in the list.
The implementation uses an internal index value that specifies where the cycle begins (if one exists). The last node's next pointer will connect back to the node at position index. When index is -1, the list ends normally with null and has no cycle.
Important: The index value is used internally and is not passed to your function.
Constraints:
- 1 ≤ Length of the list ≤ 1000
- -1000 ≤ Node.val ≤ 1000
- index is -1 or a valid index in the linked list
Examples:
// Example 1:
// List: 1 -> 2 -> 3 -> 4
// ^---------/
const head1 = createCycleList([1, 2, 3, 4], 1);
console.log(hasCycle(head1));
// Output: true
// Explanation: There is a cycle, where the tail connects to index 1
// Example 2:
// List: 1 -> 2 -> null
const head2 = createCycleList([1, 2], -1);
console.log(hasCycle(head2));
// Output: false
// Explanation: No cycle exists
