and which one is that?

void std::reverse<class Iterator>(Iterator begin, Iterator end) is an in-place iterator-based swap and overkill because it makes sense only in one of the two contexts:

1. string s1 = to_string(x); string s2 = s1 /* copy */; reverse(s2.begin(), s2.end()); if (s1 == s2) ...

2. as above but you use a function that converts an int to a vector<int> where each element is a digit then do the same process as with the strings

Both are horrendous overkill compared to the multiply-modulo-add-divide loops in other answers.

static int beautifulDays(int i, int j, int k) {

    int count = 0;
    for(int m = i ; m <= j ; m++){
        int reverse = reverseNumber(m);
        if((Math.abs(reverse - m) % k) == 0){
            count++;
        }
    }

    return count;

}

static int reverseNumber(int number){
    Stack<Integer> digits = new Stack<Integer>();
    while(number > 0){
        int remainder = number % 10;
        digits.push(remainder);
        number /= 10;
    }
    int reversedNumber = 0;
    int count = 0;
    while(!(digits.isEmpty())){
        reversedNumber += digits.pop() * Math.pow(10, count);
        count += 1;
    }

    return reversedNumber;
}

int beautifulDays(int i, int j, int k) { int num,count=0,gn; int quo, newNum = 0, x = 0; for (int x = i; x <= j ; x++) { num = x; //make sure to assign this newNum = 0; //For reversing no while (num != 0) { quo = num % 10; newNum = 10 * newNum + quo; num = num / 10; } gn = abs(x- newNum); if (gn%k==0) { count++; } } return count; }

the correct code for reversing (from above commented code)

num=x //assignment of the temp number while(num!=0) { //code } if u dont assign a new variable(num) that leads to changing the value of for loop initiailizer (x) to 0 at end of the loop ,which makes the loop infinite.