int maxSum(vector<int> &a) {
    int n = a.size();
    if (n == 0)
        return 0;
    if (n == 1)
        return a[0];
    vector<int> dp(n);
    dp[0] = a[0];
    dp[1] = max(a[0], a[1]);
    for (int i = 2; i < n; ++i)
        dp[i] = __________________________;
    return dp[n - 1];
}