【LeetCode】025. Reverse Nodes in k-Group

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

k is a positive integer and is less than or equal to the length of the linked 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

题解：

　　反转链表最适合递归去解决。首先想到的是判断当前链表是否有k个数可以反转，如果没有k个数则返回头结点，否则反转该部分链表并递归求解后来的反转链表部分。为数不多的Hard 题AC

Solution 1

/**
 * 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* cur = head, *tail = nullptr;
        for(int i = 0; i < k; ++i) {
            if (!cur)
                return head;
            if (i == k - 1)
                tail = cur;
            cur = cur->next;
        }
        
        reverse(head, tail);
        head->next = reverseKGroup(cur, k);
        
        return tail;
    }
    
    void reverse(ListNode* head, ListNode* tail) {
        if (head == tail)
            return;
        ListNode* pre = head, *cur = pre->next;
        while(cur != tail) {
            ListNode* tmp = cur->next;
            cur->next = pre;
            pre = cur;
            cur = tmp;
        }
        cur->next = pre;
    }
};

　　简化版：

class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        ListNode *cur = head;
        for (int i = 0; i < k; ++i) {
            if (!cur) return head;
            cur = cur->next;
        }
        ListNode *new_head = reverse(head, cur);
        head->next = reverseKGroup(cur, k);
        return new_head;
    }
    ListNode* reverse(ListNode* head, ListNode* tail) {
        ListNode *pre = tail;
        while (head != tail) {
            ListNode *t = head->next;
            head->next = pre;
            pre = head;
            head = t;
        }
        return pre;
    }
};

转自：Grandyang

　　首先遍历整个链表确定总长度，如果长度大于等于k，那么开始反转这k个数组成的部分链表。交换次数为k-1次，反转后，更新剩下的总长度，迭代进行。

Solution 2

/**
 * 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 dummy(-1);
        dummy.next = head;
        ListNode* pre = &dummy, *cur = pre;
        int len = 0;
        while (cur->next) {
            ++len;
            cur = cur->next;
        }
        while (len >= k) {
            cur = pre->next;
            for (int i = 0; i < k - 1; ++i) {
                ListNode* tmp = cur->next;
                cur->next = tmp->next;
                tmp->next = pre->next;
                pre->next = tmp;
            }
            pre = cur;
            len -= k;
        }
        
        return dummy.next;
    }
};