Im trying to swap position of two node in doubly linked list without swapping the data, this is my code, it came up with wrong answer when run through this testcase:

the list length: 20

the list: 2158 2398 300 2268 3655 765 3792 4038 1761 4762 1292 3200 3882 962 488 1938 3757 3122 302 640

positions to swap: 9 12

the right answer is: 2158 2398 300 2268 3655 765 3792 4038 3200 1292 4762 1761 3882 962 488 1938 3757 3122 302 640

but mine got a bit different:2158 2398 300 2268 3655 765 3792 4038 3200 4762 1292 1761 3882 962 488 1938 3757 3122 302 640. I need some help here

/*struct ListNode {
        int val;
        ListNode *left;
        ListNode *right;
        ListNode(int x = 0, ListNode *l = nullptr, ListNode* r = nullptr) : val(x), left(l), right(r) {}
    };
    */
    
    ListNode* reverse(ListNode* head, int a, int b) {
        //To Do
        if(a==b) return head;
        ListNode* h = head;
        ListNode* anode = new ListNode;
        ListNode* bnode = new ListNode;
        int n(1);
        for(int i(1); h!= nullptr; i  ){
            if(i == a){
                anode = h;
                
            }
            if(i == b){
                bnode = h;
                // break; // now h = bnode
            }
              n;
            h= h->right;
        }
        ListNode* bleft = new ListNode;
        ListNode* bright = new ListNode;
        bright = bnode->right;
        bleft = bnode->left;
        
        bnode->right = anode->right;
        bnode->right->left = bnode;
        if(a==1){
            
        }
        else{
            bnode->left = anode->left;
            bnode->left->right = bnode;
        }
        
        anode->left = bleft;
        bleft->right = anode;
        if(bright ==nullptr){
            anode->right = nullptr;
            
        }
        else{
            anode->right = bright;
            bright->left = anode;
        }
        if(a==1) return bnode;
        return head;
    }

CodePudding user response：

The right answer, as well as the name of the function you are supposed to write, suggest that you should reverse a sublist. Your code doesn't attempt to do that. It swaps positions of the first and the last nodes of the sublist. These are two different operations. The reverse operation generally needs a loop to iterate over the nodes of the sublist.

On an unrelated note, you code has four totally useless new expressions. They contribute nothing but memory leaks. Just remove them.