POI Task Chain

#include <iostream>
#include <vector>

using namespace std;

struct BigInt {
    static const int MX = 1000;
    int v[MX]{};
    BigInt operator + (const BigInt &b) {
        BigInt c;
        for (int i = 0; i < MX; ++i) c.v[i] = v[i] + b.v[i];
        for (int i = MX-2; i >= 0; --i) {
            c.v[i] += c.v[i+1] / 10;
            c.v[i+1] %= 10;
        }
        return c;
    }
    BigInt (int x) { // x is a digit
        v[MX-1] = x;
    }
    BigInt () {}
    void print() {
        bool seen = false;
        for (int i = 0; i < MX; ++i) {
            if (v[i]) seen = true;
            if (seen) cout << v[i];
        }
        if (!seen) cout << 0;
        cout << endl;
    }
};

int main() {
    int n; cin >> n;
    vector<int> a(n);
    for (int &x : a) cin >> x;
    vector<bool> change(n);
    for (int i = n-1; i >= 0; --i) {
        if (a[i] == 1) {
            change[i] = true;
            if (i) a[i-1] = 1-a[i-1];
        }
    }

    BigInt sum, pow2(1);
    for (int i = 0; i < n; ++i) {
        if (change[i]) sum = sum + pow2;
        pow2 = pow2 + pow2;
    }
    sum.print();
    return 0;
}