Add AddDigits task solution

Add Youtube link to CheckCapitalUsage video

Author: andrei-punko

README.md

@@ -175,3 +175,4 @@ they contain enough code which describes implementation in a natural way.
 - [Длина последнего слова в строке (3 решения) (leetcode)](https://www.youtube.com/watch?v=Kev5TpsfKT4)
 - [Парсинг списка строк в структуру данных (Яндекс)](https://www.youtube.com/watch?v=RW1DcmbzbQ8)
 - [Перемещение нулей к концу массива (leetcode)](https://youtu.be/IV5EIDRYZ9U)
+- [Проверка правильности использования заглавных букв (2 решения) (leetcode)](https://youtu.be/v0EkBQbFQpk)

src/main/java/by/andd3dfx/numeric/AddDigits.java

@@ -0,0 +1,55 @@
+package by.andd3dfx.numeric;
+
+/**
+ * <pre>
+ * https://leetcode.com/problems/add-digits/description/
+ *
+ * Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.
+ *
+ * Example 1:
+ *
+ * Input: num = 38
+ * Output: 2
+ * Explanation: The process is
+ * 38 --> 3 + 8 --> 11
+ * 11 --> 1 + 1 --> 2
+ * Since 2 has only one digit, return it.
+ *
+ * Example 2:
+ *
+ * Input: num = 0
+ * Output: 0
+ * </pre>
+ */
+public class AddDigits {
+
+    public static int addDigits(int num) {
+        if (num < 10) {
+            return num;
+        }
+
+        var sum = 0;
+        while (num > 0) {
+            sum += num % 10;
+            num /= 10;
+        }
+
+        return addDigits(sum);
+    }
+
+    /**
+     * https://en.wikipedia.org/wiki/Digital_root
+     */
+    public static int addDigits_usingDigitalRoot(int num) {
+        if (num == 0) {
+            return 0;
+        }
+
+        var remainder = num % 9;
+        if (remainder == 0) {
+            return 9;
+        }
+
+        return remainder;
+    }
+}

src/test/java/by/andd3dfx/numeric/AddDigitsTest.java

@@ -0,0 +1,28 @@
+package by.andd3dfx.numeric;
+
+import org.junit.Test;
+
+import java.util.function.Function;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class AddDigitsTest {
+
+    @Test
+    public void testAddDigits() {
+        makeTestsNCheckAssertions(AddDigits::addDigits);
+    }
+
+    @Test
+    public void testAddDigits_usingDigitalRoot() {
+        makeTestsNCheckAssertions(AddDigits::addDigits_usingDigitalRoot);
+    }
+
+    private static void makeTestsNCheckAssertions(Function<Integer, Integer> func) {
+        for (int i = 0; i <= 9; i++) {
+            assertThat(func.apply(i)).isEqualTo(i);
+        }
+        assertThat(func.apply(15)).isEqualTo(6);
+        assertThat(func.apply(38)).isEqualTo(2);
+    }
+}