Second commit

ReversePolishNotation/src/com/rpn/ReversePolishNotation.java

@@ -6,7 +6,9 @@
 package com.rpn;
 
 import java.math.BigDecimal;
+import java.util.ArrayDeque;
 import java.util.Arrays;
+import java.util.Deque;
 import java.util.List;
 
 /**
@@ -16,6 +18,8 @@ import java.util.List;
 public class ReversePolishNotation 
 {
     
+    private static final String OPERATORS = "+-*/";
+    
     public static BigDecimal evalRPN(String[] params)
     {
         return evalRPN(Arrays.asList(params));
@@ -26,7 +30,42 @@ public class ReversePolishNotation
         BigDecimal result = null;
         if (null != params && !params.isEmpty())
         {
+            Deque<String> stack = new ArrayDeque<>();
             
+            for (String param : params)
+            {
+                if (!OPERATORS.contains(param))
+                    stack.push(param);
+                else
+                {
+                    BigDecimal b = new BigDecimal(stack.pop());
+                    BigDecimal a = new BigDecimal(stack.pop());
+                    BigDecimal res = null;
+                    
+                    switch (param)
+                    {
+                        case "+":
+                            res = a.add(b);
+                            break;
+                        case "-":
+                            res = a.subtract(b);
+                            break;
+                        case "*":
+                            res = a.multiply(b);
+                            break;
+                        case "/":
+                            res = a.divide(b);
+                            break;
+                    }
+                    
+                    if (null != res)
+                        stack.push(String.valueOf(res));
+                    else
+                        throw new IllegalStateException("Invalid operator");
+                }
+            }
+            
+            result = new BigDecimal(stack.pop());
         }
         return result;
     }