合并两个排序的链表

Question

I am trying to write a program to merge two sorted linked lists.But the last element does not get inserted at all. The code seems to fail when try to insert a node after the last node in listA. Any help would be appreciated.

  /*
      Merge two sorted lists A and B as one linked list
      Node is defined as 
      struct Node
      {
         int data;
         struct Node *next;
      }
    */
    Node* MergeLists(Node *headA, Node* headB)
    {
      // This is a "method-only" submission. 
      // You only need to complete this method 
      Node *prev;
      Node *currentA=headA;
      Node *currentB=headB;
      Node *next;
        if(headA==NULL)
            {
            return headB;
        }
        if(headB==NULL)
            {
            return headA;
        }
      while(currentA!=NULL && currentB!=NULL)
          {
        if(currentB->data < currentA->data)
            {
            Node *new_node=new Node;
            new_node->data=currentB->data;
            headA=new_node;
            new_node->next=currentA;
            currentA=new_node;
            currentB=currentB->next;
        }
        else if(currentB->data > currentA->data && currentB->data < currentA->next->data)
            {
            Node *new_node=new Node;
            new_node->data=currentB->data;
            next=currentA->next;
            prev=currentA;
            new_node->next=next;
            prev->next=new_node;
            currentA=new_node;
            currentB=currentB->next;

        }
        else
            {
            currentA=currentA->next;

        }
          if(currentA->next==NULL)
              {
              Node *new_node=new Node;
              new_node->data=currentB->data;
              prev=currentA;
              prev->next=new_node;
              new_node->next=NULL;
              currentA=new_node;
              currentB=currentB->next;
          }
      }
        return headA;
    }

Input:

1 (No of test cases)
2 (Length of list A)
2 4
4 (Length of list B)
1 3 5 7

Expected output:

1 2 3 4 5

Actual Output:

Runtime error

Answer 1

I think the real problem is with your input.

1 (No of test cases)

2 (Length of list A)

2 4

2 (Length of list B)

1 3 5

The length of list B should be 3. As it is mentioned 2 in your input. Your output contains only 2 elements from your list B.

Kindly check with the correct input which is as follows:

1 (No of test cases)

2 (Length of list A)

2 4

3 (Length of list B)

1 3 5

Answer 2

When the code reaches the end of list A or list B, it's missing the code to copy the remainder of list A or list B. I'm also wondering about the check using currentA->next after setting currentA = currentA->next, which could be NULL.

The code is also complex, if (currentB->data < currentA->data), you move or copy a node from list B, else you move or copy a node from list A, there's no need for the complex else if after if (currentB->data < currentA->data), just the else is good enough.

The code could be simplified:

    prev = NULL;    /* this will be sorted list */
    /* ... */
        if(prev == NULL){     /* if first element */
            prev = new_node;
            next = new_node;
        } else {
            next->next = new_node;
            next = new_node;
        }
    /* ... */
    next->next = NULL;        /* set end of list */

You might want to consider changing the name of prev to head and next to tail.

Usually a mergelists function moves nodes from list A and list B into the sorted output list, as opposed to creating a list of copies of the nodes. In this case, new Node would never be used, and the existing nodes from list A and list B would be linked together in order.