单链表判断是否有环和环起始节点

package acmcoder;

class Node_t{
    int value;
    Node_t next;
    public Node_t(int value, Node_t next) {
        this.value = value;
        this.next = next;
    }
    public Node_t() {
        value = -999;
        next = null;
    }

    @Override
    public String toString() {
        return "Node_t: value=" + value;
    }


}

public class Test_didi {
    private boolean hasLoop(Node_t head){
        //使用快慢指针
        Node_t p = head.next;
        Node_t q = head.next;
        boolean res = false;
        while(q != null && q.next != null){
            p = p.next; 
            q = q.next.next;
            if(p == q){
                res = true;
                break;
            }
        }
        return res;
    }

    private Node_t crashNode(Node_t head){
        //假定一定有环
        //使用快慢指针
        Node_t p = head.next;
        Node_t q = head.next;
        while(q.next != null){
            p = p.next;
            q = q.next.next;
            if(p == q){
                break;
            }
        }
        return p;
    }

    private Node_t findCircleStartNode(Node_t head){
        boolean hasCircle = hasLoop(head);
        if(!hasCircle){
            return new Node_t();
        }else{
            //碰撞点到连接点的距离等于头指针到连接点的距离
            Node_t crash = crashNode(head);
            Node_t start = head.next;
            while(start != crash){
                start = start.next;
                crash = crash.next;
            }
            return crash;
        }
    }

    private int circleLen(Node_t head){
        //找到碰撞点之后从碰撞点出发然后再次碰撞
        Node_t crash = crashNode(head);
        Node_t p = crash;
        Node_t q = crash;
        int len = 0;
        while(q.next != null){
            p = p.next;
            q = q.next.next;
            len++;
            if(p == q){
                break;
            }
        }
        return len;
    }

    public static void main(String[] args) {
        //建立单链表
        Node_t node1 = new Node_t(1, null);
        Node_t node2 = new Node_t(2, null);
        Node_t node3 = new Node_t(3, null);
        Node_t node4 = new Node_t(4, null);
        Node_t node5 = new Node_t(5, null);
        Node_t node6 = new Node_t(6, null);
        node1.next = node2;
        node2.next = node3;
        node3.next = node4;
        node4.next = node5;
        node5.next = node6;
        node6.next = node4;
        //判断是否有环
        Test_didi test = new Test_didi();
        boolean hs = test.hasLoop(node1);
        System.out.println("Has circle: " + hs);
        //找出环的起点
        Node_t circleStart = test.findCircleStartNode(node1);
        System.out.println("Circle starts: " + circleStart.toString());
        //计算环的长度
        int len = test.circleLen(node1);
        System.out.println("Circle length: " + len);

    }

}