Question

I have tried to implement Singly Linked List. My addAtLast() function is not getting executed properly. Program crashes while executing this function. Kindly suggest some changes.

class LList
{
public:
    int noOfNodes;
    Node const *start;/*Header Node*/

    LList()
    {
        start=new Node;
        noOfNodes=0;start=0;
    }

    void addAtFront(Node* n)
    {
        /*
        cout<<endl<<"class"<<n;
        cout<<"start"<<start;
        cout<<"data in node";n->print();
        */
        n->next=const_cast<Node*>(start);
        start=n;
        // cout<<" next=";start->print();
        noOfNodes++;
    }

    void addAtLast(Node* n)
    {
        Node *cur=const_cast<Node*>(start);
        if (start==NULL)
        { 
            start=n;
            return;
        }
        while(cur->next!=NULL)
        {
            cur=cur->next;
        }
        cur->next=n;
        noOfNodes++;
    }

    int getPosition(Node data)
    {
        int pos=0;
        Node *cur=const_cast<Node*>(start);
        while(cur!=NULL)
        {
            pos++;
            if(*cur==data)
            {
                return pos;
            }
            cur=cur->next;
        }
        return -1;//not found
    }

    Node getNode(int pos)
    {
        if(pos<1)
            return -1;// not a valid position
        else if(pos>noOfNodes)
            return -1; // not a valid position

        Node *cur=const_cast<Node*>(start);
        int curPos=0;
        while(cur!=NULL)
        {
            if(++curPos==pos)
                return *cur;
            cur=cur->next;
        }
    }

    void traverse()
    {
        Node *cur=const_cast<Node*>(start);
        while(cur!=NULL)
        {
            //   cout<<"start"<<start;        
            cur->print();
            cur=cur->next;
        }
    }  

    ~LList()
    {
        delete start;
    }
};

Answer 1

I mentioned this in-comment, but will address it here as an answer. The caller of this function must ensure two things:

1. That the passed-in node list (and it is a list, even if only one element long) must be properly terminated with an end-next-pointer set to NULL. The caller must ensure this, as this code cannot assume it and blindly set node->next = NULL;

2. Make absolutely sure the caller is aware that once this executes, this list now owns the passed-in node and any list it potentially starts and the caller, therefore, must NOT free it, or anything it points to, on the caller side.

Apart from the node-count management issue, there is nothing wrong with the addAtLast() function, though I would have implemented it a little differently:

void addAtLast(Node* n)
{
    // no nulls allowed
    if (n == NULL)
        return;

    // set start if first in list
    if (start == NULL)
    {
        start = n;
        noOfNodes = 1;
    }

    // else walk list to find end
    else
    {
        Node *cur = const_cast<Node*>(start);
        while(cur->next != NULL)
            cur = cur->next;
        cur->next = n;
        ++noOfNodes;
    }

    // adjust count to contain any nodes from 'n'
    while (n->next != NULL)
    {
        ++noOfnodes;
        n = n->next;
    }
}