[leetcode]Linked List Cycle

Linked List Cycle

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

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

算法

思路1：

快慢指针，当两个指针相遇，则表示有环，否则则无环，可能需要多遍遍历，空间复杂度O(1)，时间复杂度O(n)

 1 public class Solution {
 2     public boolean hasCycle(ListNode head) {
 3         if(head == null) return false;
 4         ListNode fast = head;
 5         ListNode slow = head;
 6         while(true){
 7             if(fast.next == null || fast.next.next == null) return false;
 8             fast = fast.next.next;
 9             slow = slow.next;
10             if(fast == slow) return true;
11         }
12     }
13 }

 

思路2：

hash表，当某个节点有两个前驱，则该节点是环的起点，而且只需一次遍历，空间复杂度O(n)，时间复杂度O(n)

 1 public class Solution {
 2     public boolean hasCycle(ListNode head) {
 3         if(head == null) return false;
 4         ListNode tem = new ListNode(0);
 5         tem.next = head;
 6         Map<ListNode,ListNode> hash = new HashMap<ListNode,ListNode>();
 7         while(true){
 8             if(tem.next == null) return false;
 9             if(hash.get(tem.next) != null) return true;
10             hash.put(tem.next,tem);
11             tem = tem.next;
12         }
13     }
14 }

思路２同样是Linked List Cycle II的解法

 

ListNode的结构如下：

1 public class ListNode {
2     int val;
3     ListNode next;
4     ListNode(int x){
5         val = x;
6         next = null;
7     }
8 }
View Code

posted on 2014-07-19 14:59  喵星人与汪星人  阅读(293)  评论(0)  编辑  收藏  举报