Backtracking: Subsets

One correct implementation. Alternative approaches that satisfy the constraints are equally valid — the reviewer judges behaviour, not style.

#include <bits/stdc++.h>
using namespace std;

void backtrack(int i, vector<int>& a, vector<int>& cur, vector<vector<int>>& out) {
  if (i == (int)a.size()) { out.push_back(cur); return; }
  backtrack(i + 1, a, cur, out);
  cur.push_back(a[i]);
  backtrack(i + 1, a, cur, out);
  cur.pop_back();
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  int n; cin >> n;
  vector<int> a(n); for (int i = 0; i < n; i++) cin >> a[i];
  vector<vector<int>> out; vector<int> cur;
  backtrack(0, a, cur, out);
  cout << out.size() << '\n';
  return 0;
}