快慢指针，当两指针相遇时，再另其中一个指针指向头结点，并让两指针以同一速度遍历指针，相遇时必然在入口处

/**
 * 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) {
        ListNode fast = head, slow = head;
        while (fast != null && slow != null) {
            slow = slow.next;
            fast = fast.next;
            if (fast != null)
                fast = fast.next;
            if (fast == slow) {
                fast = head;
                while (fast != slow) {
                    fast = fast.next;
                    slow = slow.next;
                }
                return fast;
            }
        }
        return null;
    }
}