Getting segmentation error in merging two sorted linked list

[Devatha Naga Puneeth]

Given two sorted linked lists consisting of N and M nodes respectively. The task is to merge both of the list (in-place) and return head of the merged list.
Note: It is strongly recommended to do merging in-place using O(1) extra space.
QUESTION URL:https://practice.geeksforgeeks.org/problems/merge-two-sorted-linked-lists/1

My solution in C++:(This is only the main code )

struct Node {
  int data;
  struct Node *next;
  
  Node(int x) {
    data = x;
    next = NULL;
  }
};


Node* sortedMerge(Node* head_A, Node* head_B)  
{  
    Node* third = NULL;
    Node* last  = NULL;
    if(head_A->data < head_B->data)
    {
        third  = head_A;
        last   = head_A;
        head_A = head_A->next;
        third->next = NULL;
    }
    else
    {
        third  = head_B;
        last   = head_B;
        head_B = head_B->next;
        third->next = NULL;
    }
    
    
    while(head_A && head_B)
    {
        if(head_A->data < head_B->data)
        {
            last->next = head_A;
            last = head_A;
            head_A = head_A->next;
            last->next = NULL;
        }
        if(head_A->data >  head_B->data)
        {
            last->next = head_B;
            last = head_B;
            head_B = head_B->next;
            last->next = NULL;
        }
        
    }
    if(head_A)
        {
            last->next = head_A;
        }
        if(head_B)
        {
            last->next = head_B;
        }
    return third;
}  

I am getting segmentation error when i was submitted and i want know the test case where i am getting segmentation error