算法练习(12) - 判断单链表是否有环

public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        int i = 1;
        while(fast != null ){
            i = -i;
            if(i>0){
                slow = slow.next;
            }
            fast = fast.next;
            if(slow == fast){
                return true;
            }
        }
        return false;
    }
}