金九银十之两个链表的相交的第一个节点

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文字题解

找到第一个节点要分有环跟无环两种情况

代码实现

private static class Node {
    public int value;
    public Node next;

    public Node(int value) {
        this.value = value;
    }

}
private static Node getIntersectNode(Node head1, Node head2) {
    if (head1 == null || head2 == null) {
        return null;
    }
    Node loop1 = getLoopNode(head1);
    Node loop2 = getLoopNode(head2);
    if (loop1 == null && loop2 == null) {
        return noLoop(head1, head2);
    }
    if (loop1 != null && loop2 != null) {
        return bothLoop(head1, loop1, head2, loop2);
    }
    return null;
}

/**
 * 两个无环链表,返回第一个相交节点,如果不相交就返回null
 * @param head1
 * @param head2
 * @return
 */
private static Node noLoop(Node head1, Node head2) {
    if (head1 == null || head2 == null) {
        return null;
    }
    Node cur1 = head1;
    Node cur2 = head2;
    int n = 0;
    while (cur1.next != null) {
        n++;
        cur1 = cur1.next;
    }
    while (cur2.next != null) {
        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;
}

/**
 * 两个有环链表,返回第一个相交节点,如果不相交就返回null
 *
 * @param head1
 * @param loop1
 * @param head2
 * @param loop2
 * @return
 */
private static Node bothLoop(Node head1, Node loop1, Node head2, Node loop2) {
   Node cur1 = null;
    Node cur2 = null;
    if (loop1 == loop2) {
        cur1 = head1;
        cur2 = head2;
        int n = 0;
        while (cur1 != loop1) {
            n++;
            cur1 = cur1.next;
        }
        while (cur2 != loop2) {
            n--;
            cur2 = cur2.next;
        }
        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;
    } else {
        cur1 = loop1.next;
        while (cur1 != loop1) {
            if (cur1 == loop2) {
                return loop1;
            }
            cur1 = cur1.next;
        }
        return null;
    }
}

/**
 * @param head 找到入环节点
 * @return
 */
public static Node getLoopNode(Node head) {
   Node n1 = head.next; // n1 -> slow
    Node n2 = head.next.next; // n2 -> fast
    while (n1 != n2) {
        if (n2.next == null || n2.next.next == null) {
            return null;
        }
        n2 = n2.next.next;
        n1 = n1.next;
    }
    n2 = head; // n2 -> walk again from head
    while (n1 != n2) {
        n1 = n1.next;
        n2 = n2.next;
    }
    return n1;
}