检查LinkedList中的循环

//tc :O(N) N is the length of the linkedList
//sc:(1) constant space complexity
/**
 * 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) {
        //we can use double jump to find if loop is present in the loop
        if(head ==null || head.next ==null) return false;
        ListNode node1 = head;
        ListNode node2 = head.next;
        while(node1!=null && node2!=null){
            if(node1==node2) return true;
            node1 = node1.next;
            node2  = node2.next ==null ? null : node2.next.next;
        }
        return false;
    }
}