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

 """
 Definition of ListNode
 class ListNode(object):

     def __init__(self, val, next=None):
         self.val = val
         self.next = next
 """
 class Solution:
     """
     @param head: The first node of the linked list.
     @return: True if it has a cycle, or false
     """
     def hasCycle(self, head):
         # write your code here
         if head == None or head.next == None:
             return False
         slow = head
         fast = head
         fast = fast.next
         while fast.next!=None and fast.next.next!=None:
             if fast == slow:
                 return True
             else:
                 slow = slow.next
                 fast = fast.next.next
         return False 

Python Code