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 slow=head;
        ListNode fast=head;
        boolean flag=false;
        while(fast!=null && fast.next!=null){
            slow=slow.next;
            fast=fast.next.next;
            if(fast==null) break;
            if(fast==slow) {
                flag=true;
                break;
            }
        }
        return flag;
    }
}

思路：快行指针。