super digit

README.md

@@ -40,6 +40,8 @@ This repo register my evolution in the Cracking The Coding Interview book as I g
 
 [Find ZigZag Sequence](https://github.com/brendonmiranda/CrackingTheCodingInterview/blob/main/src/main/java/hackerRank/week2/FindZigZagSequence.java)
 
+[Super Digit]()
+
 # AWS Challenges
 
 [Review Score](https://github.com/brendonmiranda/CrackingTheCodingInterview/blob/main/src/main/java/aws/ReviewScore.java)

src/main/java/hackerRank/week2/SuperDigit.java

@@ -0,0 +1,39 @@
+public class SuperDigit {
+
+    public static int superDigit(String n, int k) {
+
+        LinkedList<Integer> list = new LinkedList<>();
+
+        for (int i=0; i< n.length(); i++) {
+            list.add(Integer.valueOf(String.valueOf(n.charAt(i))));
+        }
+
+        Long sum = 0L; // huge numbers
+        boolean firstLoop = true;
+
+        while(!list.isEmpty()) {
+            int i =list.pop();
+            sum+=i;
+
+            if(list.isEmpty()) {
+                if(firstLoop) {
+                    sum = sum * k; // avoid concatenation 
+                    firstLoop = false;                    
+                }
+
+                if(sum<10) {
+                    return sum.intValue();
+                }
+
+                String v = String.valueOf(sum);
+                for (int j=0; j<v.length(); j++) {
+                    list.add(Integer.valueOf(String.valueOf(v.charAt(j))));
+                }
+                sum = 0L;
+            }
+        }
+
+        return -1;
+    }
+
+}