141. 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?

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