Ý tưởng : sử dụng bit 

#include <bits/stdc++.h>
using namespace std;
int main()
{
  freopen("chonso.inp" , "r" , stdin);
  freopen("chonso.out" , "w" , stdout);
  long long n, m;
  cin >> n;
  vector<long long> a(n), r(n);
  for (long long i = 0; i < n; ++i) cin >> a[i];
  cin >> m;
  for (long long i = 0; i < 1 << n; ++i)
  {
    long long s = 0;
    vector<long long> b(n);
    for (long long j = 0; j < n; ++j)
    {
      if (i & (1 << j))
      {
        s += a[j];
        b[j] = 1;
      }
      else 
      {
        b[j] = 0;
      }
    }
    if (s == m)
    {
      r = b;
    }
  }
  for (long long i: r)
  {
    cout << i;
  }
}

#danglam228

#include <bits/stdc++.h>
using namespace std;
int n, m;
int main() {
        ios_base::sync_with_stdio(false);
    cin.tie(0); cout.tie(0);
    freopen("CHONSO.INP", "r", stdin);
    freopen("CHONSO.OUT", "w", stdout);
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }
    cin >> m;

    vector<vector<int>> f(n + 1, vector<int>(m + 1, 0));
    vector<vector<int>> g(n + 1, vector<int>(m + 1, 0));

    f[0][0] = 1;

    for (int i = 1; i <= n; ++i) {
        for (int j = 0; j <= m; ++j) {
            if (f[i-1][j]) {
                f[i][j] = f[i-1][j];
                g[i][j] = 0;
            }
            if (j >= a[i-1] && f[i-1][j - a[i-1]]) {
                f[i][j] = 1;
                g[i][j] = 1;
            }
        }
    }

    string st(n, '0');
    int d = m;
    for (int i = n; i > 0; --i) {
        if (g[i][d]) {
            st[i-1] = '1';
            d -= a[i-1];
        }
    }

    cout << st << endl;

    return 0;
}