lintcode:带环链表

带环链表

给定一个链表，判断它是否有环。

解题

定义两个指针p1 p2

p1每次向前走一步

p2每次向前走两步

当p2能赶上p1的时候说明有环

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The first node of linked list.
     * @return: True if it has a cycle, or false
     */
    public boolean hasCycle(ListNode head) {  
        // write your code here
        if(head == null || head.next == null)
            return false;
        ListNode p1 = head;
        ListNode p2 = head;
        p2 = p2.next;
        while(p2.next!=null && p2.next.next!=null){
            if(p1 == p2){
                return true;
            }
            p1 = p1.next;
            p2 = p2.next.next;
        }
        return false;
    }
}

Java Code

 1 """
 2 Definition of ListNode
 3 class ListNode(object):
 4 
 5     def __init__(self, val, next=None):
 6         self.val = val
 7         self.next = next
 8 """
 9 class Solution:
10     """
11     @param head: The first node of the linked list.
12     @return: True if it has a cycle, or false
13     """
14     def hasCycle(self, head):
15         # write your code here
16         if head == None or head.next == None:
17             return False
18         slow = head
19         fast = head
20         fast = fast.next
21         while fast.next!=None and fast.next.next!=None:
22             if fast == slow:
23                 return True
24             else:
25                 slow = slow.next
26                 fast = fast.next.next
27         return False

Python Code