2180

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

@@ -57,6 +57,37 @@ public:
 };
 ```
 
+## Solution 2. Pattern Finding
+
+`num` | 1 |2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|
+--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--|--
+digit sum |1|2|3|4|5|6|7|8|9|1|2|3|4|5|6|7|8|9|10|2|3|4|5|6|7
+digit sum is even|0|1|0|1|0|1|0|1|0|0|1|0|1|0|1|0|1|0|0|1|0|1|0|1|0
+answer |0|1|1|2|2|3|3|4|4|4|5|5|6|6|7|7|8|8|9|10|10|11|11|12|12|
+
+
+The answer increments when the digit sum of `num` is even.
+
+We can see that the answer is somewhat related to `num / 2` and `digitSum(num) % 2`. It turns out to be `(num - digitSum(num) % 2) / 2`.
+
+```cpp
+// OJ: https://leetcode.com/problems/count-integers-with-even-digit-sum/
+// Author: github.com/lzl124631x
+// Time: O(lgN)
+// Space: O(1)
+class Solution {
+public:
+    int countEven(int num) {
+        int sum = 0, tmp = num;
+        while (num) {
+            sum += num % 10;
+            num /= 10;
+        }
+        return (tmp - sum % 2) / 2;
+    }
+};
+```
+
 ## Discuss
 
 https://leetcode.com/problems/count-integers-with-even-digit-sum/discuss/1784735