Test56.java

/**
 * Author: 王俊超
 * Date: 2015-06-16
 * Time: 08:49
 * Declaration: All Rights Reserved !!!
 */
public class Test56 {
    private static class ListNode {
        private int val;
        private ListNode next;

        public ListNode() {
        }

        public ListNode(int val) {
            this.val = val;
        }

        @Override
        public String toString() {
            return val +"";
        }
    }

    public static ListNode meetingNode(ListNode head) {

        ListNode fast = head;
        ListNode slow = head;

        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                break;
            }
        }

        // 链表中没有环
        if (fast == null || fast.next == null) {
            return null;
        }

        // fast重新指向第一个结点
        fast = head;

        while (fast != slow) {
            fast = fast.next;
            slow = slow.next;
        }

        return fast;
    }

    public static void main(String[] args) {
        test01();
        test02();
        test03();
    }

    // 1->2->3->4->5->6
    private static void test01() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(2);
        ListNode n3 = new ListNode(3);
        ListNode n4 = new ListNode(4);
        ListNode n5 = new ListNode(5);
        ListNode n6 = new ListNode(6);

        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;

        System.out.println(meetingNode(n1));
    }

    // 1->2->3->4->5->6
    //       ^        |
    //       |        |
    //       +--------+
    private static void test02() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(2);
        ListNode n3 = new ListNode(3);
        ListNode n4 = new ListNode(4);
        ListNode n5 = new ListNode(5);
        ListNode n6 = new ListNode(6);

        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n3;

        System.out.println(meetingNode(n1));
    }

    // 1->2->3->4->5->6 <-+
    //                |   |
    //                +---+
    private static void test03() {
        ListNode n1 = new ListNode(1);
        ListNode n2 = new ListNode(2);
        ListNode n3 = new ListNode(3);
        ListNode n4 = new ListNode(4);
        ListNode n5 = new ListNode(5);
        ListNode n6 = new ListNode(6);

        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n6;

        System.out.println(meetingNode(n1));
    }
}