cashier.cpp

#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    int answer = n; /// assume we used all of coin1
    for (int c4 = 0; c4 <= n / 4; c4++) { // try to take c4 of coin4
        for (int c3 = 0; c3 <= (n - 4 * c4) / 3; c3++) { // try to take c3 of coin3
            int c1 = n - (4 * c4 + 3 * c3);
            answer = min(answer, c1 + c3 + c4);
        }
    }
    cout << answer << endl;
    return 0;
}

count-digits.cpp

#include <bits/stdc++.h>
using namespace std;
 
int count_digits_string(long long n){
	string s = to_string(n);
	return s.size();
}
int count_digits_loop(long long n){
	int answer = 0;
	if(n == 0)
		return 1;
	else if(n < 0)// count negative sign and make n positive
		answer = 1, n *= -1;
	while(n > 0)// while not zero count last digit and remove it
		answer++, n /= 10;
	return answer;
}
 
int count_digits_math(long long n){
	int answer = 0;
	if(n < 0)// count negative sign and make n positive
		answer++, n *= -1;
	answer += log10(n) + 1;// log10(n) + 1 is the number of digits in n
	return answer;
}
 
int main(){
	long long a, b;/// use long long to avoid overflow
	cin >> a >> b;
	cout << count_digits_math(a*b);
	return 0;
}

hardest-problem.c

#include <stdio.h>

int main () {
  int n; 
  scanf("%d", &n); // cin >> n;
  printf("%d\n", n); // cout << n << endl;
  return 0;
}

sort-my-marks-v2.cpp

#include <bits/stdc++.h>

using namespace std;

double calc(string mark)
{
    int i = mark.find_first_of('/');
    int got = stoi(mark.substr(0, i));
    double total = stoi(mark.substr(i + 1, mark.size() - i + 1));
    return got / total;
}

char rnk(double mark)
{
    if (mark >= .85)
        return 'A';
    if (mark < .85 && mark >= .7)
        return 'B';
    if (mark < .7 && mark >= .65)
        return 'C';
    if (mark < .65 && mark >= .5)
        return 'D';

    return 'E';
}

struct Subject
{
    string name;
    double percent;
};

bool subj_sort(const Subject &sub1, const Subject &sub2)
{
    return sub1.percent > sub2.percent;
}

int main()
{
    int n;
    cin >> n;
    Subject subjects[n];
    string mark;
    for (int i = 0; i < n; i++)
    {
        cin >> subjects[i].name >> mark;
        subjects[i].percent = calc(mark);
    }

    sort(subjects, subjects + n, subj_sort);

    for (int i = 0; i < n; i++)
    {
        cout << subjects[i].name << " " << rnk(subjects[i].percent) << endl;
    }
}

sort-my-marks.cpp

#include <bits/stdc++.h>

using namespace std;

char rnk(double mark)
{
    if (mark >= .85)
        return 'A';
    if (mark < .85 && mark >= .7)
        return 'B';
    if (mark < .7 && mark >= .65)
        return 'C';
    if (mark < .65 && mark >= .5)
        return 'D';
    else
        return 'E';
}

int main()
{
    int n;
    cin >> n;
    string subjects[n];
    string mark;
    double percent[n];
    for (int i = 0; i < n; i++)
    {
        cin >> subjects[i];
        cin >> mark;

        // now let's process the string mark
        int scored = 0;
        double total = 0;
        string st_scored = "", st_total = "";
        int index = 0;
        while (mark[index] != '/')
            st_scored += mark[index], index++;
        // now, index is pointing to '/' char
        // let's push index one step towards the first digit
        index++;
        // now, index is pointing to the first digit after '/'
        while (index < mark.size())
            st_total += mark[index], index++;

        scored = stoi(st_scored); // stoi = string to integer
        total = stod(st_total);   // stod = string to double
        percent[i] = scored / total;
    }

    // the following is a simple sorting algorithm
    // selection sort
    for (int i = 0; i < n; i++)
    {
        double max = percent[i];
        int index = i;
        for (int j = i + 1; j < n; j++)
        {
            if (percent[j] > max)
            {
                max = percent[j];
                index = j;
            }
        }
        swap(percent[i], percent[index]);
        swap(subjects[i], subjects[index]);
    }

    for (int i = 0; i < n; i++)
    {
        cout << subjects[i] << " " << rnk(percent[i]) << endl;
    }
}

sum-them-all.cpp

#include <bits/stdc++.h>

using namespace std;

int main()
{
    string s;
    int x;
    cin >> s >> x;
    int operations = 0; // initially we didn't perform any operations yet
    
    // first check if S is same as X
    if (s == to_string(x)) // convert x to a string and compare for equality
    {
        cout << operations << endl;
        return 0;
    }

    while (s.size() > 1) // as long as the number of digits in S is greater than 1
    {
        operations++;

        int sum = 0;

        for (char c : s) // for every char in S
        {
            sum += c - '0'; // convert character to int 
        }

        s = to_string(sum);

        if (x == sum)
        {
            cout << operations << endl;
            return 0;
        }
    }

    if (s == to_string(x))
        cout << operations << endl;
    else
        cout << -1 << endl;
}