141.环形链表

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/**
 * 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) {
        if(head == null || head.next == null) return false;

        ListNode slow = head;
        ListNode fast = head.next;

        while(slow != fast) {
            if(fast == null || fast.next == null) return false;
            slow = slow.next;
            fast = fast.next.next;
        }

        return true;
    }
}