// 方法：获取环的起始点。参数length表示环的长度
    public Node getCycleStart(Node head, int cycleLength) {
        if (head == null) {
            return null;
        }
        Node first = head;
        Node second = head;
        // 先让second指针走length步 
        for (int i = 0; i < cycleLength; i++) {
            second = second.next;
        }
        // 然后让first指针和second指针同时各走一步
        while (first != null && second != null) {
            first = first.next;
            second = second.next;
            if (first == second) {
                return first;
            }
        }
        return null;
    }

public class LinkCycleStart {
    public Node head;
    public Node current;

    // 向链表中添加数据
    public void add(int data) {
        // 判断链表为空的时候
        if (head == null) {// 如果头结点为空，说明这个链表还没有创建，那就把新的结点赋给头节点
            head = new Node(data);
            current = head;
        } else {
            current.next = new Node(data);// 创建新的结点，放在当前节点的后面（把新的节点和链表进行关联）
            current = current.next;// 把链表的当前索引向后移动一位，此步操作完成之后，current结点指向新添加的那个结点
        }
    }

    // 方法重载：向链表中添加结点
    public void add(Node node) {
        if (node == null) {
            return;
        }
        if (head == null) {
            head = node;
            current = head;
        } else {
            current.next = node;
            current = current.next;
        }
    }

    // 方法：遍历链表（打印输出链表。方法的参数表示从节点node开始进行遍历
    public void print(Node node) {
        if (node == null) {
            return;
        }
        current = node;
        while (current != null) {
            System.out.println(current.data);
            current = current.next;
        }
    }

    class Node {
        // 注：此处的两个成员变量权限不能为private，因为private的权限是仅对本类访问
        int data;// 数据域
        Node next;// 指针域

        public Node(int data) {
            this.data =http://www.mamicode.com/ data;
        }

        public int getData() {
            return data;
        }

        public void setData(int data) {
            this.data =http://www.mamicode.com/ data;
        }

        public Node getNext() {
            return next;
        }

        public void setNext(Node next) {
            this.next = next;
        }
    }

    // 方法：检测单链表是否有环
    public Node hasCycle(Node head) {
        if (head == null) {
            return null;
        }
        Node first = head;
        Node second = head;
        while (second != null) {
            first = first.next;
            second = second.next.next;
            if (first == second) {// 一旦两个指针相遇，说明链表是有环的
                return first;
            }
        }
        return null;

    }

    // 方法：有环链表中，获取环的长度。参数node代表的是相遇的那个结点
    public int getCycleLength(Node node) {
        if (head == null) {
            return 0;
        }
        current = node;
        int length = 0;
        while (current != null) {
            current = current.next;
            length++;
            if (current == node) {
                return length;// 当current结点走到原点的时候
            }
        }
        return length;
    }

    // 方法：获取环的起始点。参数length表示环的长度
    public Node getCycleStart(Node head, int cycleLength) {
        if (head == null) {
            return null;
        }
        Node first = head;
        Node second = head;
        // 先让second指针走length步 
        for (int i = 0; i < cycleLength; i++) {
            second = second.next;
        }
        // 然后让first指针和second指针同时各走一步
        while (first != null && second != null) {
            first = first.next;
            second = second.next;
            if (first == second) {
                return first;
            }
        }
        return null;
    }

    public static void main(String[] args) {
        LinkCycleStart list1 = new LinkCycleStart();
        Node second = null; // 把第二个结点记下来
        // 向链表中添加数据
        for (int i = 0; i < 4; i++) {
            list1.add(i);
            if (i == 2) {
                second = list1.current;// 把第二个结点记下来
            }
        }

        list1.add(second);// 将尾结点指向链表的第二个结点，于是单链表就有环了
        Node current1 = list1.hasCycle(list1.head);// 获取相遇的那个结点
        int length = list1.getCycleLength(current1);
        System.out.println("环的起始点是" + list1.getCycleStart(list1.head, length).data);
    }

}