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

Example 1:

Input: num = 4
Output: 2
Explanation:
The only integers less than or equal to 4 whose digit sums are even are 2 and 4.    

Example 2:

Input: num = 30
Output: 14
Explanation:
The 14 integers less than or equal to 30 whose digit sums are even are
2, 4, 6, 8, 11, 13, 15, 17, 19, 20, 22, 24, 26, and 28.

Constraints:

• 1 <= num <= 1000

Solution: Brute Force

Use std::to_string to convert an integer to string.

Time complexity: O(nlgn)
Space complexity: O(lgn)

// Author: Huahua
class Solution {
public:
  int countEven(int num) {
    int ans = 0;
    for (int i = 1; i <= num; ++i) {
      int sum = 0;
      for (char c : to_string(i))
        sum += (c - '0');
      if (sum % 2 == 0) ++ans;
    }
    return ans;
  }
};

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