Reverse Nodes in k-Group_recursion.cpp

/**
 * 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) {
        if(k <= 1)return head;
        return reverse(head, k);
    }

private:
    ListNode* reverse(ListNode* head, int k)
    {
        if(!head)return head;
        int i = k - 1;
        ListNode* end = head;
        while(i-- && end)end = end->next;
        if(!end)return head;
        ListNode* curr = head->next, *prev = head;
        while(prev != end)
        {
            ListNode* tmp = curr->next;
            curr->next = prev;
            prev = curr;
            curr = tmp;
        }
        head->next = reverse(curr, k);
        return prev;
    }
};