微软: Reverse Linked List II

/**
 * 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) {
        //check the input 
        if ( head == nullptr) return nullptr;
        if (head->next == nullptr) return head;
        
        ListNode* headnode = new ListNode(0);
        headnode -> next = head;
        
        int i = 1;
        ListNode* first = headnode;
        ListNode* next = headnode;
        ListNode* cur = headnode;
        
        while (i < m){//next node should be reversed that is m
            first = first ->next;
            i++;
        }
        
        ListNode* temp = first->next;
        next = first->next->next;
        i+=1;
        
        while (i <=n){
            if (next == nullptr) break;
            cur = next;
            next = next->next;
            cur->next = first->next;
            first->next = cur;
            i++;
        }
        
        temp->next = next;
        head = headnode->next;
        delete headnode ;
        return head;
    }
};