[LintCode] 102 Linked List Cycle 解题报告

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



Example
Given -21->10->4->5, tail connects to node index 1, return true



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



思路
如果这个链表有环，那么如果两个人在环里走，一个快一个慢，快的人多跑一圈以后就会追上并且和慢的人重合。



Code

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