141.环形链表

给定一个链表，判断链表中是否有环。

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        boolean result = false;
        while(fast != null) {
            if(fast.next != null) {
                fast = fast.next.next;
            } else {
                break;
            }
                
            slow = slow.next;
            if(fast == slow) {
                result = true;
                break;
            }
        }
        return result;
    }
}