/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
class Solution {
    public ListNode entryNodeOfLoop(ListNode head) {
        if(head == null){return null;}
        ListNode fast = head;
        ListNode slow = head;
        while(fast.next.next != null && slow.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if(fast.val == slow.val){
                slow = head;
                while(fast.next != null && slow.next != null){
                    fast = fast.next;
                    slow = slow.next;
                    if(fast.val == slow.val){
                        return slow;
                    }
                }
            }
        }
        return null;
    }
}