How to fix the runtime error(SIGSEGV) in this code

I am trying to represent a graph through adjacency list .

I have declared an array of pointers and then inserted elements in a linked list style.

#include<iostream>
using namespace std;

struct node
{
    int data;
    node* next;
};

int main()
{
    int testcases,vertex,edge;
    cin>>testcases>>vertex>>edge;
    node* arr[1001];
    for(int i=0;i<1001;++i)
    arr[i]=NULL;
    while(testcases--)
    {
        while(edge--)
        {
            int a,b;
            cin>>a>>b;
            node* p=arr[a];
            if(!p)
            {
                p=new node;
                p->data=b;
                p->next=NULL;
            }
            else
            {
                while(!(p->next))
                p=p->next;
                node*t=new node;
                t->data=b;
                t->next=NULL;
                p->next=t;
            }
            for(int i=0;i<vertex;++i)
            {
                cout<<i<<"->"<<" ";
                node* t=arr[i];
                while(t->next!=NULL)
                {
                    cout<<t->data<<"->"<<" ";
                }
                cout<<endl;
            }
        }
    }
}

SIGSEGV error occurs because of accessing of invalid index . But I'm not able to find out where the out-of-bound access is occurring .