链表-返回链表第一个进入环的节点

//返回链表第一个进入环的节点
public class Node{
    public int value;
    public Node next;
    public Node(int data){
        this.data=data;
    }
}
public Node getLoopNode(Node head){
    if(head==null||head.next==null||head.next.next==null){
        return null;
    }
    Node node1=head.next;
    Node node2=head.next.next;
    while(node1!=node2){
        if(node1.next==null||node2.next.next==null){
            return null;
        }
        node2=node2.next.next;
        node1=node1.next;
    }
    node2=head;//node2->walk again from head 
    while(node1!=node2){
        node1=node1.next;
        node2=node2.next;
    }
    return node1;
}