Refactored all solutions

Java/Data Structures/Java Stack/Solution.java

@@ -3,11 +3,12 @@
 // HackerRank: hackerrank.com/rshaghoulian
 
 import java.util.Scanner;
-import java.util.Stack;
 import java.util.HashMap;
+import java.util.ArrayDeque;
+
+// ArrayDeque is "likely to be faster than Stack when used as a stack" - Java documentation
 
 class Solution {
-    
     public static void main(String[] args) {
         /* Create HashMap to match opening brackets with closing brackets */
         HashMap<Character, Character> map = new HashMap<>();
@@ -24,21 +25,19 @@ class Solution {
         scan.close();
     }
     
-    public static boolean isBalanced(String expression, HashMap<Character, Character> map) {
-        /* Odd length strings are not balanced */
+    private static boolean isBalanced(String expression, HashMap<Character, Character> map) {
         if ((expression.length() % 2) != 0) {
-            return false;
-        }
-        
-        Stack<Character> stack = new Stack<>();
+            return false; // odd length Strings are not balanced
+        }        
+        ArrayDeque<Character> deque = new ArrayDeque<>(); // use deque as a stack
         for (int i = 0; i < expression.length(); i++) {
             Character ch = expression.charAt(i);
             if (map.containsKey(ch)) {
-                stack.push(ch);
-            } else if (stack.isEmpty() || ch != map.get(stack.pop())) {
+                deque.push(ch);
+            } else if (deque.isEmpty() || ch != map.get(deque.pop())) {
                 return false;
             }
         }
-        return stack.isEmpty();
+        return deque.isEmpty();
     }
 }