题解 | #判断链表中是否有环#

/**
 * 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 fast = new ListNode(-1);
        ListNode slow = fast;
        slow.next = head;
	  // 快指针比慢指针多走一步，如果重叠，则意味着有环，否则，则没有环
        while(fast!=null && fast.next!=null){
            fast= fast.next.next;
            slow = slow.next;
            if(fast == slow){
                return true;
            }
        }
        return false;
    }
}