Linked List Cycle | Leetcode

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

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Solution: Maintain two pointers, slow and fast. Advance slow by one node at a time. Advance fast by two nodes at a time. If slow and fast ever points to the same node, then there is a cycle. If we reach the end of the linked list, then there is no cycle.

/**
 * 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;
            fast = fast.next.next;
            if (slow == fast) return true;
        }
        return false;
    }
}