题目

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.

If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.

You may not alter the values in the nodes, only nodes itself may be changed.

Only constant memory is allowed.

For example,
Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5

分析

该题目核心是分组对一个链表进行反转，链接，如示例所示。

AC代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        ListNode *p = head;
        int len = 0;
        while (p)
        {
            len++;
            p = p->next;
        }
        if (len < k || k == 1)
            return head;

        vector<ListNode *> vl;
        ListNode *h = head;

        while (len >= k)
        {
            //找到len/k个子链表，分别翻转
            ListNode *q = h;
            for (int i = 1; i < k; i++)
                q = q->next;

            //保存后续结点
            ListNode *r = q->next;

            q->next = NULL;

            vl.push_back(reverse(h));

            h = r;
            len -= k;
        }//while

        //将翻转的链表链接起来,保存剩余的小于k个的结点
        ListNode *re = h;
        if (vl.size() != 1)
        {

            for (int i = 0; i < vl.size() - 1; i++)
            {
                p = LastNode(vl[i]);
                p->next = vl[i + 1];
            }//for
            p->next = vl[vl.size() - 1];
        }

        LastNode(vl[vl.size() - 1])->next = re;

        head = vl[0];
        return head;

    }

    ListNode *LastNode(ListNode *head)
    {
        while (head && head->next)
            head = head->next;
        return head;
    }

    ListNode* reverse(ListNode* head)
    {
        if (head == NULL || head->next == NULL)
            return head;

        ListNode *ret = head , *p = head->next;
        ret->next = NULL;

        while (p)
        {
            //保存下一个结点
            ListNode *q = p->next;

            p->next = ret;
            ret = p;
            p = q;
        }
        return ret;
    }
};

GitHub测试程序源码