solves #2243: Calculate Digit Sum of a String in java

Author: anishLearnsToCode

README.md

@@ -729,7 +729,7 @@
 | 2235 | [Add Two Integers](https://leetcode.com/problems/add-two-integers)                                                                                                     |                                                                                       [![Java](assets/java.png)](src/AddTwoIntegers.java)                                                                                        |                                                                                                                                   |
 | 2236 | [Root Equals Sum of Children](https://leetcode.com/problems/root-equals-sum-of-children)                                                                               |                                                                                   [![Java](assets/java.png)](src/RootEqualsSumOfChildren.java)                                                                                   |                                                                                                                                   |
 | 2239 | [Find Closest Number to Zero](https://leetcode.com/problems/find-closest-number-to-zero)                                                                               |                                                                                   [![Java](assets/java.png)](src/FindClosestNumberToZero.java)                                                                                   |                                                                                                                                   |
-| 2243 | [Calculate Digit Sum of a String](https://leetcode.com/problems/calculate-digit-sum-of-a-string)                                                                       |                                                                                                                                                                                                                                  |                                                                                                                                   |
+| 2243 | [Calculate Digit Sum of a String](https://leetcode.com/problems/calculate-digit-sum-of-a-string)                                                                       |                                                                                 [![Java](assets/java.png)](src/CalculateDigitSumOfAString.java)                                                                                  |                                                                                                                                   |
 | 2248 | [Intersection of Multiple Arrays](https://leetcode.com/problems/intersection-of-multiple-arrays)                                                                       |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 2255 | [Count Prefixes of a Given String](https://leetcode.com/problems/count-prefixes-of-a-given-string)                                                                     |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 2259 | [Remove Digit From Number to Maximize Result](https://leetcode.com/problems/remove-digit-from-number-to-maximize-result)                                               |                                                                                                                                                                                                                                  |                                                                                                                                   |

src/CalculateDigitSumOfAString.java

@@ -0,0 +1,28 @@
+// https://leetcode.com/problems/calculate-digit-sum-of-a-string
+// T: O(log(n))
+// S: O(log(n))
+
+public class CalculateDigitSumOfAString {
+    public String digitSum(String s, int k) {
+        if (s.length() <= k) return s;
+        final StringBuilder result = new StringBuilder();
+        for (int i = 0 ; i < s.length() ; i += k) {
+            final String group = s.substring(i, Math.min(i + k, s.length()));
+            final int groupSum = digitSum(group);
+            result.append(groupSum);
+        }
+        return digitSum(result.toString(), k);
+    }
+
+    private int digitSum(String number) {
+        int sum = 0;
+        for (int index = 0 ; index < number.length() ; index++) {
+            sum += toInt(number.charAt(index));
+        }
+        return sum;
+    }
+
+    private int toInt(char x) {
+        return x - '0';
+    }
+}