count-integers-with-even-digit-sum.cpp

// Time:  O(logn)
// Space: O(1)

// math
class Solution {
public:
    int countEven(int num) {
        const auto& parity = [](int x) {
            int result = 0;
            for (; x; x /= 10) {
                result += x % 10;
            }
            return result % 2;
        };
        return (num - parity(num)) / 2;
    }
};

// Time:  O(logn)
// Space: O(1)
// math
class Solution2 {
public:
    int countEven(int num) {
        const auto& parity = [](int x) {
            int result = 0;
            for (; x; x /= 10) {
                result += x % 10;
            }
            return result % 2;
        };
        int result = 0;
        for (int i = 1; i <= num; ++i) {
            result += static_cast<int>(parity(i) == 0);
        }
        return result;
    }
};