Reverse Linked List II.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        ListNode* helper = new ListNode(-1);
        helper->next = head;
        ListNode* curr = helper;
        for(int i = 0; i < m - 1; ++i)
            curr = curr->next;
        curr->next = reverse(curr->next, n - m);
        return helper->next;
    }
    
private:
    ListNode* reverse(ListNode* head, int k)
    {
        ListNode* curr = head->next, *prev = head;
        while(curr && k--)
        {
            ListNode* tmp = curr->next;
            curr->next = prev;
            prev = curr;
            curr = tmp;
        }
        head->next = curr;
        return prev;
    }
};