Help in RPLA(Spoj) Topological Sort Question

I have just studied kahn’s algorithm for topological sort and while practicing, I was not able to get AC in RPLA(Spoj).

Here is my code. It passes the example test cases but getting WA after submitting.

#include <algorithm>
#include <iostream>
#include <climits>
#include <cstring>
#include <string>
#include <vector>
#include <queue>
#include <cmath>
#include <set>
#include <map>

using namespace std;

#define all(x) (x).begin(),(x).end()
#define ll long long
#define mod 1000000007
#define vi vector<int>
#define vll vector<ll>
#define pb push_back

ll power(int x, unsigned int y){
    ll res = 1;
    while(y > 0){
        if(y & 1) res = res * x;
        y >>= 1;
        x *= x;
    }
    return res;
}

vi arr[1000];
int in[1000];

struct myCmp {
    bool operator() (pair<int, int> a, pair<int,int> b) {
        if(a.first < b.first) {
            return a.first < b.first;
        }else return a.second < b.second;
    }
};

void kahn(int nodes) {

    priority_queue<pair<int, int> , vector<pair<int,int> >, myCmp > q;

    int start = 1;
    for(int i = 0; i < nodes; i++) {
        if(in[i] == 0) {
            q.push({start, i});
            cout << start << ' ' << i << '\n';
        }
    }
    start++;
    while(!q.empty()) {
        int curr = q.top().second;
        q.pop();
        bool check = false;
        for(int child : arr[curr]) {
            in[child]--;
            if(in[child] == 0) {
                check = true;
                q.push({start, child});
                cout << start << ' ' << child << '\n';
            }
        }
        if(check) start++;
    }
}

void solve() {

    for(int i = 0; i < 1000; i++) arr[i].clear(), in[i] = 0;

    int n, m;
    cin >> n >> m;

    while(m--) {
        int a, b;
        cin >> b >> a;
        arr[a].pb(b);
        in[b]++;
    }

    kahn(n);

}

int main(){

    #ifndef ONLINE_JUDGE
    freopen("/ATOMCODES/input.txt", "r", stdin);
    freopen("/ATOMCODES/output.txt", "w", stdout);
    #endif

    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int t; cin >> t;
    for(int i = 1; i <= t; i++) {
        cout << "Scenario #" << i << ':'<< '\n';
        solve();
    }

}