edit 150

src/main/java/com/fishercoder/solutions/_150.java

@@ -62,4 +62,30 @@ public int evalRPN(String[] tokens) {
         return Integer.parseInt(stack.pop());
     }
 
+    //using one stack only.
+    public int evalRPN1(String[] tokens) {
+        Stack<Integer> stack =  new Stack<Integer>();
+        for (int i = 0; i < tokens.length; i++) {
+            if (tokens[i].equals("+")){
+                stack.push(stack.pop() + stack.pop());
+            }
+            else if (tokens[i].equals("-")){
+                stack.push(-stack.pop() + stack.pop());
+            }
+            else if (tokens[i].equals("*") ){
+                stack.push(stack.pop() * stack.pop());
+
+            }
+            else if (tokens[i].equals("/")){
+                int a = stack.pop();
+                int b = stack.pop();
+                stack.push(b / a);
+
+            } else {
+                stack.push(Integer.parseInt (tokens[i]));
+            }
+        }
+        return stack.pop();
+    }
+
 }