B. Максимизируй сумму цифр

main.cc

#include <bits/stdc++.h>

using namespace std;

using ll = long long;

ll GetNum(vector<int>& digits) {
    while (!digits.empty() && digits[0] == 0) digits.erase(begin(digits));
    ll ans = 0;
    for (auto digit: digits) ans = ans * 10 + digit;
    return ans;
}

ll GetSum(vector<int>& digits) {
    ll ans = 0;
    for (auto x: digits) ans += x;
    return ans;
}

vector<int> ToDigits(ll num) {
    vector<int> digits;
    while (num != 0) {
        digits.push_back(num % 10);
        num /= 10;
    }
    if (digits.empty()) digits.push_back(0);
    reverse(begin(digits), end(digits));
    return digits;
}

int main(int argc, char **argv) {
    ios::sync_with_stdio(false);
    srand(time(0));

    ll n;
    cin >> n;
    vector<int> digits = ToDigits(n);
    ll max_sum = GetSum(digits);
    ll ans = n;
    for (int i = (int) digits.size() - 1; i > 0; --i) {
        if (digits[i] == 9) continue;
        int to = i - 1;
        while (to >= 0 && digits[to] == 0) digits[to--] = 9;
        if (to < 0) break;
        --digits[to];
        digits[i] = 9;
        if (GetSum(digits) > max_sum) {
            max_sum = GetSum(digits);
            ans = GetNum(digits);
        }
    }
    cout << ans << endl;
    return 0;
}