2180

leetcode/2180. Count Integers With Even Digit Sum/README.md

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+# [2180. Count Integers With Even Digit Sum (Easy)](https://leetcode.com/problems/count-integers-with-even-digit-sum/)
+
+<p>Given a positive integer <code>num</code>, return <em>the number of positive integers <strong>less than or equal to</strong></em> <code>num</code> <em>whose digit sums are <strong>even</strong></em>.</p>
+
+<p>The <strong>digit sum</strong> of a positive integer is the sum of all its digits.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre><strong>Input:</strong> num = 4
+<strong>Output:</strong> 2
+<strong>Explanation:</strong>
+The only integers less than or equal to 4 whose digit sums are even are 2 and 4.    
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre><strong>Input:</strong> num = 30
+<strong>Output:</strong> 14
+<strong>Explanation:</strong>
+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.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= num &lt;= 1000</code></li>
+</ul>
+
+
+**Similar Questions**:
+* [Sum of Digits of String After Convert (Easy)](https://leetcode.com/problems/sum-of-digits-of-string-after-convert/)
+
+## Solution 1. Brute Force
+
+```cpp
+// OJ: https://leetcode.com/contest/weekly-contest-281/problems/count-integers-with-even-digit-sum/
+// Author: github.com/lzl124631x
+// Time: O(NlgN)
+// Space: O(1)
+class Solution {
+public:
+    int countEven(int num) {
+        int ans = 0;
+        for (int i = 1; i <= num; ++i) {
+            int n = i, sum = 0;
+            while (n) {
+                sum += n % 10;
+                n /= 10;
+            }
+            ans += sum % 2 == 0;
+        }
+        return ans;
+    }
+};
+```
+
+## Discuss
+
+https://leetcode.com/problems/count-integers-with-even-digit-sum/discuss/1784735