solves reverse polish notation in java

Author: anishLearnsToCode

Diff for: README.md

@@ -133,7 +133,7 @@
 | 146  | [LRU Cache](https://leetcode.com/problems/lru-cache)                                                                                                      |                                                                    [![Java](assets/java.png)](src/LRUCache.java)                                                                     |                                                                                                                                   |
 | 147  | [Insertion Sort List](https://leetcode.com/problems/insertion-sort-list)                                                                                  |                                                                [![Java](assets/java.png)](src/InsertionSortList.java)                                                                |                                                                                                                                   |
 | 148  | [Sort List](https://leetcode.com/problems/sort-list)                                                                                                      |                                                                    [![Java](assets/java.png)](src/SortList.java)                                                                     |                                                                                                                                   |
-| 150  | [Evaluate Reverse Polish Notation](https://leetcode.com/problems/evaluate-reverse-polish-notation)                                                        |                                                                                                                                                                                      |                                                                                                                                   |
+| 150  | [Evaluate Reverse Polish Notation](https://leetcode.com/problems/evaluate-reverse-polish-notation)                                                        |                                                          [![Java](assets/java.png)](src/EvaluateReversePolishNotation.java)                                                          |                                                                                                                                   |
 | 151  | [Reverse Words in a String](https://leetcode.com/problems/reverse-words-in-a-string)                                                                      |                                                                                                                                                                                      |                                                                                                                                   |
 | 152  | [Maximum Product Subarray](https://leetcode.com/problems/maximum-product-subarray)                                                                        |                                                                                                                                                                                      |                                                                                                                                   |
 | 153  | [Find Minimum in Rotated Sorted Array](https://leetcode.com/problems/find-minimum-in-rotated-sorted-array)                                                |                                                                                                                                                                                      |                                                                                                                                   |

Diff for: src/EvaluateReversePolishNotation.java

@@ -0,0 +1,48 @@
+// https://leetcode.com/problems/evaluate-reverse-polish-notation
+// T: O(N)
+// S: O(N)
+
+import java.util.Set;
+import java.util.Stack;
+
+public class EvaluateReversePolishNotation {
+    private static final String ADDITION = "+";
+    private static final String SUBTRACTION = "-";
+    private static final String MULTIPLICATION = "*";
+    private static final String DIVISION = "/";
+
+    private static final Set<String> OPERATORS = Set.of(
+        ADDITION,
+        SUBTRACTION,
+        MULTIPLICATION,
+        DIVISION
+    );
+
+    public static int evalRPN(String[] tokens) {
+        Stack<Integer> stack = new Stack<>();
+        for (String token : tokens) {
+            if (isOperator(token)) {
+                int number1 = stack.pop();
+                int number2 = stack.pop();
+                stack.push(apply(number2, number1, token));
+            } else {
+                stack.push(Integer.parseInt(token));
+            }
+        }
+        return stack.pop();
+    }
+
+    private static boolean isOperator(String token) {
+        return OPERATORS.contains(token);
+    }
+
+    private static int apply(int a, int b, String operator) {
+        return switch (operator) {
+            case ADDITION -> a + b;
+            case SUBTRACTION -> a - b;
+            case MULTIPLICATION -> a * b;
+            case DIVISION -> a / b;
+            default -> 0;
+        };
+    }
+}

Diff for: src/HelloWorld.java

@@ -1,4 +1,7 @@
 public class HelloWorld {
     public static void main(String[] args) {
+        System.out.println(Integer.parseInt("2"));
+        System.out.println(Integer.parseInt("-11"));
+        System.out.println(EvaluateReversePolishNotation.evalRPN(new String[] {"2", "-11", "-"}));
     }
 }