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. 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).

Example 2:

Input: head = [1,2], pos = 0

Output: true

Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Example 3:

Input: head = [1], pos = -1

Output: false

Explanation: There is no cycle in the linked list.

 

Constraints:

The number of the nodes in the list is in the range [0, 104].

-105 <= Node.val <= 10^5

pos is -1 or a valid index in the linked-list.

 

Follow up: Can you solve it using O(1) (i.e. constant) memory?

🔶 Step: Detect if a cycle exists

Use the slow and fast pointer logic:

• slow = head, fast = head

• Move:

  • slow = slow.next

  • fast = fast.next.next

• If they meet, a cycle exists

/**

 * Definition for singly-linked list.

 * class ListNode {

 *     int val;

 *     ListNode next;

 *     ListNode(int x) {

 *         val = x;

 *         next = null;

 *     }

 * }

 */

public class Solution {

    public boolean hasCycle(ListNode head) {

    ListNode slow = head;

    ListNode fast = head;

    while (fast != null && fast.next != null) {

        slow = slow.next;            // 1 step

        fast = fast.next.next;       // 2 steps

        if (slow == fast) {

            return true; // Cycle detected

        }

    }

    return false; // No cycle

    }

}