Create add-digits.cpp

Author: kamyu104

C++/add-digits.cpp

@@ -0,0 +1,9 @@
+// Time:  O(1)
+// Space: O(1)
+
+class Solution {
+public:
+    int addDigits(int num) {
+        return (num - 1) % 9 + 1;
+    }
+};