链表中环的入口结点

/*
 public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}
*/
import java.util.ArrayList;
public class Solution {

    public ListNode EntryNodeOfLoop(ListNode pHead)
    {
        ArrayList<ListNode> list = new ArrayList<>();
        ListNode node = pHead;
        while(node != null){
            if(list.contains(node)){
                return node;
            }else{
                list.add(node);
                node = node.next;
            }
        }
        return null;
    }
}

另一种不需要空间复杂度为O(1)的

 if(pHead == null || pHead.next == null || pHead.next.next == null){
            return null;
        }
        ListNode slow = pHead.next;
        ListNode fast = pHead.next.next;
        while(fast != slow){
            if(fast.next == null || fast.next.next == null){
                return null;
            }
            fast = fast.next.next;
            slow = slow.next;
        }
        //得到环的长度
        ListNode mid = pHead;
        fast = fast.next;
        while(fast != slow){
            fast = fast.next;
            mid = mid.next;
        }
        mid = mid.next;
        fast = pHead;
        while(fast != mid){
            fast = fast.next;
            mid = mid.next;
        }
        return fast;