Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

分析

这题用hashset很容易

如果不用set，可以用slow，fast双指针，fast走的快，如果有环的话，迟早会在后面的几圈会赶上slow。

/**
 * 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 || head.next == null){
          return false;
      }

      ListNode slow = head;
      ListNode fast = head.next;
      while(slow != fast){
          if(fast == null || fast.next == null){
              return false;
          }
          fast = fast.next.next;
          slow = slow.next;
      }  
     return true;   
    }
}