算法系列之-两个链表是否相交

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package it.list;

/**
* @项目名称：util
* @类名称：Huan @类描述：判断两个链表是否有交点
*
* @author 赵建银
* @date 2018年1月23日
* @time 下午8:05:19
* @version 1.0
*/
public class Huan {

/**
 * @param head
 * @return 获取有环链表的入环节点
 */
public static Node getLoopNode(Node head) {
    if (head == null || head.next == null || head.next.next == null) {
        return null;
    }

    Node slow = head.next;// 慢指针
    Node fast = head.next.next;// 快指针
    while (slow != fast) {
        if (slow.next == null || fast.next == null) {
            return null;
        }
        slow = slow.next;
        fast = fast.next.next;
    } // 两个相交
        // 快指针从头开始
    fast = head;
    while (fast != slow) {
        fast = fast.next;
        slow = slow.next;
    } // 和慢指针在次相交，极为入环的第一个节点
    return fast;
}

/**
 * 
 * 两个链表都没有环
 * 
 * @param head1
 * @param head2
 * @return
 */
public static Node NoLoopNode(Node head1, Node head2) {
    int n = 0;
    Node cur1 = head1;
    Node cur2 = head2;
    while (cur1.next != null) {
        n++;
        cur1 = cur1.next;
    }
    // 记录1尾节点
    while (cur2.next != null) {
        n--;
        cur2 = cur2.next;
    }
    // 记录2尾节点
    if (cur1 != cur2) {
        return null;
    }
    cur1 = n > 0 ? head1 : head2;
    cur2 = cur1 == head1 ? head2 : head1;
    n = Math.abs(n);
    // 长的链表先走n步，之后一起走
    while (n != 0) {
        n--;
        cur1 = cur1.next;
    }
    // 相交的为第一个节点
    while (cur1 != cur2) {
        cur1 = cur1.next;
        cur2 = cur2.next;
    }
    return cur1;
}

/**
 * 两个链表都有环
 * 
 * @param head1
 * @param loop1
 *            入环节点
 * @param head2
 * @param loop2
 * @return
 */
public static Node BothLoop(Node head1, Node loop1, Node head2, Node loop2) {
    Node cur1 = head1;
    Node cur2 = head2;
    if (loop1 != loop2) {// 入环节点不同
        cur1 = loop1.next;
        cur2 = loop2;
        while (cur1 != loop1) {// 第一个绕环一周
            if (cur1 == cur2) {// 发现第二个
                return loop1;// 返回任意一个
            }
            cur1 = cur1.next;
        }
        // 没有说明各自是一个环，没有焦点
        return null;
    } else {
        // 如环节点相同，在入环之前，相当于无环链表
        int n = 0;
        while (cur1.next != loop1) {
            n++;
            cur1 = cur1.next;
        }
        while (cur2.next != loop2) {
            n--;
            cur2 = cur2.next;
        }
        if (cur1 != cur2) {
            return null;
        }
        cur1 = n > 0 ? head1 : head2;
        cur2 = cur1 == head1 ? head2 : head1;
        n = Math.abs(n);
        while (n != 0) {
            n--;
            cur1 = cur1.next;
        }
        while (cur1 != cur2) {
            cur1 = cur1.next;
            cur2 = cur2.next;
        }
        return cur1;

    }
}

/**
 * 
 * 
 * @param head1
 * @param head2
 * @return 两个链表相交的第一个节点
 */
public static Node GetNode(Node head1, Node head2) {
    if (head1 == null || head2 == null) {
        return null;
    }
    Node loopNode = getLoopNode(head1);
    Node loopNode2 = getLoopNode(head2);
    Node res = null;
    if (loopNode != null && loopNode2 != null) {// 都有环
        res = BothLoop(head1, loopNode, head2, loopNode2);
    }
    if (loopNode == null && loopNode2 == null) {// 都没环
        res = NoLoopNode(head1, head2);
    }
    return res;
}

}