23-链表中环的入口节点

public class Solution {
    public ListNode EntryNodeOfLoop(ListNode pHead) {
        if (pHead == null) return null;
        Set<ListNode> set = new HashSet<>();
        ListNode tmp = pHead;
        while (tmp != null) {
            if (set.contains(tmp)) return tmp;
            set.add(tmp);
            tmp = tmp.next;
        }
        return null;
    }
}