Linked List Cycle

经典题，送分题，做过无数遍。

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null) return false;
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast) return true;
        }
        return false;
    }
}