Vector of Vectors in C++

vectors.cpp

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */ 
    int n, q;
    cin >> n >> q;
    vector<vector<int>> arr(n);

    for(int i=0; i<n; i++)
    {
        int k = 0;
        cin >> k;
        arr[i].resize(k);
        for(int j=0; j<k; j++) cin >> arr[i][j];
    }

    for(int m=0; m<q; m++)
    {
        int a = 0;
        int b = 0;
        cin >> a >> b;
        cout << arr[a][b] << endl;
    }
}