Make a Distinct Digit Array

#include<iostream>
using namespace std;

int main(){
    int t; cin >> t;
    while(t--){
        int n, i; cin >> n;
        unsigned long long int a[n];
        for(i = 0; i < n; i++) cin >> a[i];
        int digits[10] = {0};
        for(i = 0; i < n; i++){
            int k = a[i];
            while(k){
                int m = k % 10;
                digits[m] = 1;
                k /= 10;
            }
        }
        for(i = 0; i < 10; i++){
            if(digits[i] != 0) cout << i << ' ';
        }
        cout << endl;
    }
}