链表中的环的入口节点

第一次相遇说明，这个链表中有环
第二次相遇说明，这个环的入口点

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

        while(fast.next != null && fast.next.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if(slow == fast){
                ListNode k1 = head;
                while(k1 != slow){
                    k1 = k1.next;
                    slow = slow.next;
                }
                return k1;
            }
        }

        return null;
    }
}