题解 | #判断链表中是否有环#

public class Solution {
    public boolean hasCycle(ListNode head) {	
      //若头节点为空，则无环
        if(head == null) return false;
        while(head.val != 100001 && head != null){
          //依次将遍历过的节点标记
            head.val = 100001;
            head = head.next;
          //若遍历到最后一个节点则无环
            if(head == null) return false;
         }
        if(head == null) return false;
      //退出循环原因是已经遍历到标记过的节点，说明有环
        return true;
    }
}