hackerrank exercises link

Chapter 1 Arrays and strings/1.1. is_unique.md

@@ -2,7 +2,7 @@
 
 ## Implement an algorithm to determine if a S has all unique characters. What if you can't use additional data structures
 
-example: s = "abdceb"
+> example: s = "abdceb"
 
 ## 1.1.1. First idea
 

Chapter 1 Arrays and strings/arrays_strings.md

@@ -48,4 +48,8 @@ String joinWords(String[] words) {
     }
     return sentence.toString();
 }
-```
+```
+
+## Hackerrank exercises
+
+[Hackerrank string exercises](https://www.hackerrank.com/domains/algorithms?filters%5Bsubdomains%5D%5B%5D=strings)