LeetCode Linked List Cycle

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

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

/**
 * 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) {
        boolean iscycle=false;  
        if (head!=null) {
                ListNode temp=head;
                while (!iscycle) {
                    head=head.next;
                    temp.next=temp;
                    temp=head;
                    if (head==null) {
                        iscycle=false;
                        break;
                    }else {
                        if (head.next==head) {
                            iscycle=true;
                            break;
                        }
                    }
                    
                }
        }
        return iscycle;
    }
}