Add 150_Evaluate_Reverse_Polish_Notation.java

Author: Sean Prashad

Arrays/150_Evaluate_Reverse_Polish_Notation.java

@@ -0,0 +1,49 @@
+class Solution {
+    public int evalRPN(String[] tokens) {
+        Stack<Integer> st = new Stack<>();
+
+        Set<String> operators = buildOperators();
+
+        for (String token : tokens) {
+            if (!operators.contains(token)) {
+                st.push(Integer.valueOf(token));
+                continue;
+            }
+
+            Integer num2 = st.pop();
+            Integer num1 = st.pop();
+
+            int tempResult = 0;
+
+            switch (token) {
+                case "+":
+                    tempResult = num1 + num2;
+                    break;
+                case "-":
+                    tempResult = num1 - num2;
+                    break;
+                case "*":
+                    tempResult = num1 * num2;
+                    break;
+                case "/":
+                    tempResult = num1 / num2;
+                    break;
+            }
+
+            st.push(tempResult);
+        }
+
+        return st.pop();
+    }
+
+    private Set<String> buildOperators() {
+        Set<String> operators = new HashSet<>();
+
+        operators.add("+");
+        operators.add("-");
+        operators.add("*");
+        operators.add("/");
+
+        return operators;
+    }
+}