variable sized arrays hackerrank solution in c++

#include <iostream>
#include <vector>

using namespace std;

int main() {
	// get length of array 'a' and number of queries
	int n, q;
	cin >> n >> q;

	// create vector of vectors
	vector<vector<int>> a(n);

	// fill each 2D vector i with k_i values
	for (int i = 0; i < n; i++) {
		// get the length k of the vector at a[i]
		int k;
		cin >> k;

		// fill the vector with k values
		a[i].resize(k);
		for (int j = 0; j < k; j++) {
			cin >> a[i][j];
		}
	}

	// run queries on a
	for (int q_num = 0; q_num < q; q_num++) {
		// get i, j as the 'query' to get a value from a
		int i, j;
		cin >> i >> j;
		cout << a[i][j] << endl;
	}

	return 0;
}