2180. Count Integers With Even Digit Sum

Given a positive integer num, return the number of positive integers less than or equal to num whose digit sums are even.

The digit sum of a positive integer is the sum of all its digits.

class Solution {
    int sum(int n) {
        int res = 0;
        while(n) {
            res += n % 10;
            n/=10;
        }
        return res;
    }
public:
    int countEven(int num) {
        int res = 0;
        for(int i = 1; i <= num; i++) {
            int s = sum(i);
            if(s%2 == 0) res++;
        }
        return res;
    }
};