Problems and their tricks

For number of characters of each character in string ⏩ char 'c' - A'

vector<int> v(26);  // "26" alphabets in English Lnaguage
for (char c : str)
{
    v.at(c - 'A')++;
    // if "c" = 'A' then (c - 'A') => ['65' - '65' = "0"] ; as 'A' ASCII value is "65" and (c = 'A' = 65 also)
    // if "c" = 'A' then (c - 'A') => ['68' - '65' = "3"] ; as 'A' ASCII value is "65" and c = 'D' = 68
}

For converting the index created above as char ⏩ indexNo. "i" + 'A'

dq.emplace_back(i + 'A');
// index is the IndexNo. here
// If the IndexNo. "i" is 3 then then ("3" + "65") = ("68") as 'A' ASCII value is "65" ; hence 'D' = 68  ,,
// So "(i +'A') = 68"

caution

using both the above tricks:

CSES Problem: Palindrome Reader :~ Converting/Checking a String to Palindrome

#include <bits/stdc++.h>

using namespace std;
#define ll long long
#define endl "\n"

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    string str;
    cin >> str;

    vector<int> v(26);
    deque<char> dq;
    for (auto el : str)
    {
        v.at(el - 'A')++;
    }

    int count = 0;

    for (int i = 0; i < v.size(); i++)
    {
        if (v.at(i) % 2 != 0)
        {
            ++count;
        }
        if (count > 1)
        {
            cout << "NO SOLUTION" << endl;
            return 0;
        }
    }

    // making the palindrome
    // odd first added
    for (int i = 0; i < v.size(); i++)
    {
        if (v.at(i) % 2 != 0)
        {
            for (int j = 0; j < v.at(i); j++)
            {
                dq.emplace_back(i + 'A');
            }
        }
    }

    // for even
    for (int i = 0; i < v.size(); i++)
    {
        if (!(v.at(i) % 2))
        {
            // emplace_back
            for (int j = 0; j < (v.at(i) / 2); j++)
            {
                dq.emplace_back(i + 'A');
            }

            // emplace_front
            for (int j = 0; j < v.at(i) / 2; j++)
            {
                dq.emplace_front(i + 'A');
            }
        }
    }

    string ans;

    for (auto el : dq)
    {
        ans.push_back(el);
    }

    cout << ans << endl;

    return 0;
}

cycle type problems inside a loop

for(int i=0;i<100;++i) {
   pour(i%N, (i+1)%N); // Pour milk from one bucket to the next
   /* 1 ⇢ 2 ⇢ 3 ⇢ 1 */
   // and then this gets repeated again  "0%3  1%3  2%3   3%3[0 here]  4%3 5%3  6%3[0 here]  ...."
}

solution using cycle inside `loop` ⇢⇢ the question

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

vector<int> cap(3);
vector<int> am(3);

void sol(int a, int b)
{
    int x = min(cap.at(b) - am.at(b), am.at(a));
    am.at(a) -= x;
    am.at(b) += x;
}

int main()
{

    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int tmp;
    for (int i = 0; i < 3; i++)
    {
        cin >> tmp;
        cap.at(i) = tmp;
        cin >> tmp;
        am.at(i) = tmp;
    }

    for (int i = 0; i < 100; i++)
    {
        sol(i % 3, (i + 1) % 3);
    }

    for (auto el : am)
    {
        cout << el << endl;
    }

    return 0;
}