I am trying to solve merge k sorted arrays into a single array using heap

Question

I am trying to solve merge k sorted arrays into a single array using heap. But i am getting a ERROR: compiling failed with exit code 1.I don't understand what this means.

Compiling failed with exit code 1, compiler output: I am not able to understand the error I guess there is a problem with priority queue working. As i am new to this please someone suggest the changes.

I have tried using std:: pair inbuilt in stl and it works fine for that. but if i define a class like in the code it is not working

class pairr{
    public:
        int data;
        int id;
        int index;

    pairr(int data,int id,int index){
        this->data=data;
        this->id=id;
        this->index=index;
    }
};


vector<int> merge(vector<vector<int>> &V){

    priority_queue<pairr,vector<pairr>,greater<pairr>> q;

    vector<int> out;


    //creating min heap of k nodes
    for(int i=0;i<V.size();i++){
        pairr a(V[i][0],i,0);
        q.push(a);        
    }

    while(!q.empty()){
        //minimum element
        pairr cur=q.top();

        // i=array of min ele j=index of array
        int i=cur.id;
        int j=cur.index;

        //pop the element and push it in output array
        q.pop();
        out.push_back(cur.data);

        //push new element from same array
        if(j+1<V[i].size()){ 
            pairr a(V[i][j+1],i,j+1);
            q.push(a);
        }

        //return the output vector
        return out;
    }

}

int main() {
    vector<vector<int>> V={{0,4,10,12},
                 {1,3,5,7},
                 {2,4,12,15,20}};

    vector<int> output=merge(V);
    for(int i=0;i<output.size();i++){
        cout<<output[i]<<" ";
    }

    return 0;
}
```

Answer 1

You need to provide a way to compare two instances of pairr. How else would std::priority_queue know which pairr is of higher or lower priority than the other? Since you want to use greater<pairr>, you should implement operator>().

It works for std::pair because std::pair does in fact provide various comparison operators, operator> among them.