lintcode-medium-Linked List Cycle

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?

 

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