Change project structure to a Maven Java project + Refactor (#2816)

src/main/java/com/thealgorithms/maths/SumOfDigits.java

@@ -0,0 +1,60 @@
+package com.thealgorithms.maths;
+
+public class SumOfDigits {
+
+    public static void main(String[] args) {
+        assert sumOfDigits(-123) == 6 && sumOfDigitsRecursion(-123) == 6 && sumOfDigitsFast(-123) == 6;
+
+        assert sumOfDigits(0) == 0 && sumOfDigitsRecursion(0) == 0 && sumOfDigitsFast(0) == 0;
+
+        assert sumOfDigits(12345) == 15
+                && sumOfDigitsRecursion(12345) == 15
+                && sumOfDigitsFast(12345) == 15;
+    }
+
+    /**
+     * Calculate the sum of digits of a number
+     *
+     * @param number the number contains digits
+     * @return sum of digits of given {@code number}
+     */
+    public static int sumOfDigits(int number) {
+        number = number < 0 ? -number : number;
+        /* calculate abs value */
+        int sum = 0;
+        while (number != 0) {
+            sum += number % 10;
+            number /= 10;
+        }
+        return sum;
+    }
+
+    /**
+     * Calculate the sum of digits of a number using recursion
+     *
+     * @param number the number contains digits
+     * @return sum of digits of given {@code number}
+     */
+    public static int sumOfDigitsRecursion(int number) {
+        number = number < 0 ? -number : number;
+        /* calculate abs value */
+        return number < 10 ? number : number % 10 + sumOfDigitsRecursion(number / 10);
+    }
+
+    /**
+     * Calculate the sum of digits of a number using char array
+     *
+     * @param number the number contains digits
+     * @return sum of digits of given {@code number}
+     */
+    public static int sumOfDigitsFast(int number) {
+        number = number < 0 ? -number : number;
+        /* calculate abs value */
+        char[] digits = (number + "").toCharArray();
+        int sum = 0;
+        for (int i = 0; i < digits.length; ++i) {
+            sum += digits[i] - '0';
+        }
+        return sum;
+    }
+}