linked-list-cycle

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

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



/**
 * 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 slowPtr = head;
        ListNode fastPtr = head;
        while (fastPtr != null && fastPtr.next != null) {
            slowPtr = slowPtr.next;
            fastPtr = fastPtr.next.next;
            if (slowPtr == fastPtr) {
                return true;
            }
        }
        return false;
    }
}