Java——每k个结点反转一次链表

前几日，看到一道面试题，每k个结点反转一次链表，要求输出反转后的链表。

题目意思如下：

原链表：1,2,3,4,5,6,7,8

k = 3

新链表：3,2,1,6,5,4,8,7

觉得还是有点意思，今天便做了，思路是把原链表先按k切割成多份，再把每一份都反转并拼接起来。

把代码贴出来供大家参考

public class Node {
    public int v;
    public Node next;

    public Node(int v, Node next) {
        this.v = v;
        this.next = next;
    }

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

    public Node() { }

    public static Node getNode(int ... arr) {
        Node head = new Node();
        Node root = head;
        for (int i : arr) {
            head.next = new Node(i);
            head = head.next;
        }
        return root.next;
    }

    /**
     * 反转链表
     */
    public static Node reverse(Node head) {
        if (head == null) {
            return null;
        }
        Node pre = null;
        Node cur = head;
        Node aft = head.next;
        while (aft != null) {
            cur.next = pre;
            pre = cur;
            cur = aft;
            aft = aft.next;
        }
        cur.next = pre;
        return cur;
    }

    /**
     * 每k位反转
     */
    public static Node reverseK(Node head, int k) {
        List<CutRes> cutResList = new ArrayList<>();
        while (head != null) {
            CutRes cutRes = cut(head, k);
            cutResList.add(cutRes);
            head = cutRes.newHead;
        }
        Node cur = new Node();
        Node pre = cur;
        for (CutRes cutRes : cutResList) {
            cur.next = Node.reverse(cutRes.head);
            cur = cutRes.head;
        }
        return pre.next;
    }

    /**
     * 从头部切割链表
     * @param head 原头部
     * @param k 切割长度
     */
    public static CutRes cut(Node head, int k) {
        if (k <= 0) {
            return new CutRes(null, head);
        }
        Node cur = head;
        int i = 0;
        while (cur.next != null && ++i < k) {
            cur = cur.next;
        }
        Node tail = cur;
        Node newHead = cur.next;
        tail.next = null;
        return new CutRes(head, newHead);
    }

    static class CutRes {
        Node head; //切割得到的头部
        Node newHead; //切割后剩余的头部

        public CutRes(Node head, Node newHead) {
            this.head = head;
            this.newHead = newHead;
        }

        @Override
        public String toString() {
            return head.toString();
        }
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder("[");
        Node cur = next;
        sb.append(v);
        while (cur != null) {
            sb.append(", ");
            sb.append(cur.v);
            cur = cur.next;
        }
        sb.append(']');
        return sb.toString();
    }
}