`*` Số nguồn

#include <bits/stdc++.h>

using namespace std;

int sum(int n) {
    int total = 0;
    while (n) total += n % 10, n /= 10;
    return total;
}

int main()
{
    int n;
    cin >> n;
    int ans = -1;
    for (int i = 1; i <= 81; i++) {
        if (i == sum(n - i)) ans = n - i;
    }
    cout << ans;
    return 0;
}

`*` Đếm số đẹp

`-` Solution 1: For (Time: `O(n)`)

#include <bits/stdc++.h>

using namespace std;

bool Beautiful_num(int n) {
    while (n) {
        if (n % 10 != 6 && n % 10 != 8) return 0;
        n /= 10;
    }
    return 1;
}

int main()
{
    int n;
    cin >> n;
    int ans = 0;
    for (int i = 6; i <= n; i++) ans += Beautiful_num(i);
    cout << ans;
    return 0;
}

`-` Solution 2: Math + Binary Exponentiation (Time: `O(log(n) + Size(n))`)

#include <bits/stdc++.h>

using namespace std;

int Pow(int a, int n) {
    int power = 1;
    while (n) {
        if (n & 1) power *= a;
        a *= a;
        n >>= 1;
    }
    return power;
}

int Decimal(string s) {
    long long deci = 0, power = 1;
    for (int i = s.size() - 1; i >= 0; i--) {
        if (s[i] == '1') deci += power;
        power *= 2;
    }
    return deci;
}

bool Beautiful_num(int n) {
    while (n) {
        if (n % 10 != 6 && n % 10 != 8) return 0;
        n /= 10;
    }
    return 1;
}

int main()
{
    int temp;
    cin >> temp;
    while (!Beautiful_num(temp)) temp--;
    string n = to_string(temp);
    for (char &c : n) c = c == '6' ? '0' : '1';
    cout << Pow(2, n.size()) - 2 + Decimal(n) + 1;
    return 0;
}

`*` Phân tích số

#include <bits/stdc++.h>

using namespace std;

int main()
{
    int n;
    cin >> n;
    int a = 0, b = 0, ans = 1e9;
    for (long long i = 1; i * i <= n; i++) {
        if (n % i == 0) {
            if (i + n / i < ans) a = i, b = n / i, ans = a + b;
        }
    }   
    cout << a << ' ' << b;
    return 0;
}