1645 - Nearest Smaller Values

using namespace std;
using ii = pair<int, int>;
int main(void)
{
    int n, ai;
    stack<ii> S;
    cin >> n;
    for (int i = 0; i < n; i++)
    {
        cin >> ai;
        while (!S.empty() and S.top().first >= ai)
            S.pop();
        if (S.empty())
            cout << "0 ";
        else
            cout << S.top().second + 1 << " ";
        S.push({ai, i});
    }
    return 0;
}