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

Author: kamyu104

C++/count-integers-with-even-digit-sum.cpp

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+// 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;
+    }
+};