Added 26 solutions

Java/Data Structures/Java Stack/Solution.java

@@ -0,0 +1,44 @@
+//     Author: Rodney Shaghoulian
+//     Github: github.com/rshaghoulian
+// HackerRank: hackerrank.com/rshaghoulian
+
+import java.util.Scanner;
+import java.util.Stack;
+import java.util.HashMap;
+
+class Solution {
+    
+    public static void main(String[] args) {
+        /* Create HashMap to match opening brackets with closing brackets */
+        HashMap<Character, Character> map = new HashMap<>();
+        map.put('(', ')');
+        map.put('[', ']');
+        map.put('{', '}');
+        
+        /* Test each expression for validity */
+        Scanner scan = new Scanner(System.in);
+        while (scan.hasNext()) {
+            String expression = scan.next();
+            System.out.println(isBalanced(expression, map) ? "true" : "false" );
+        }
+        scan.close();
+    }
+    
+    public static boolean isBalanced(String expression, HashMap<Character, Character> map) {
+        /* Odd length strings are not balanced */
+        if ((expression.length() % 2) != 0) {
+            return false;
+        }
+        
+        Stack<Character> stack = new 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())) {
+                return false;
+            }
+        }
+        return stack.isEmpty();
+    }
+}