Add solution for Evaluate Reverse Polish Notation

Author: jihyun-um

README.md

@@ -38,6 +38,7 @@ Algorithm exercises from LeetCode implemented in Java v11.
 - Longest Absolute File Path | [Problem](https://leetcode.com/problems/longest-absolute-file-path) | [Solution](src/solutions/LongestAbsoluteFilePath.java)
 - Remove All Adjacent Duplicates In String | [Problem](https://leetcode.com/problems/remove-all-adjacent-duplicates-in-string) | [Solution](src/solutions/RemoveDuplicatesInString.java)
 - Binary Tree Inorder Traversal | [Problem](https://leetcode.com/problems/binary-tree-inorder-traversal) | [Solution](src/solutions/BinaryTreeInorderTraversal.java)
+- Evaluate Reverse Polish Notation | [Problem](https://leetcode.com/problems/evaluate-reverse-polish-notation) | [Solution](src/solutions/EvaluateReversePolishNotation.java)
 
 ### Queue
 - Queue Using Stacks | [Problem](https://leetcode.com/problems/implement-queue-using-stacks) | [Solution](src/solutions/QueueUsingStacks.java)

src/solutions/EvaluateReversePolishNotation.java

@@ -0,0 +1,52 @@
+package solutions;
+
+import java.util.*;
+
+// [Problem] https://leetcode.com/problems/evaluate-reverse-polish-notation
+class EvaluateReversePolishNotation {
+    private final Set<String> OPERATORS = Set.of("+", "-", "*", "/");
+
+    // Stack
+    // O(n) time, O(n) space
+    public int evalRPN(String[] tokens) {
+        Stack<Integer> stack = new Stack<>();
+        for (String token : tokens) {
+            if (OPERATORS.contains(token)) {
+                int num2 = stack.pop();
+                int num1 = stack.pop();
+                int result = calculate(num1, num2, token);
+                stack.add(result);
+            } else {
+                stack.add(Integer.parseInt(token));
+            }
+        }
+        return stack.pop();
+    }
+
+    private int calculate(int num1, int num2, String operator) {
+        if ("+".equals(operator)) {
+            return num1 + num2;
+        } else if ("-".equals(operator)) {
+            return num1 - num2;
+        } else if ("*".equals(operator)) {
+            return num1 * num2;
+        } else {
+            return num1 / num2;
+        }
+    }
+
+    // Test
+    public static void main(String[] args) {
+        EvaluateReversePolishNotation solution = new EvaluateReversePolishNotation();
+
+        String[] input1 = {"2", "1", "+", "3", "*"};
+        int expectedOutput1 = 9;
+        int actualOutput1 = solution.evalRPN(input1);
+        System.out.println("Test 1 passed? " + (expectedOutput1 == actualOutput1));
+
+        String[] input2 = {"10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"};
+        int expectedOutput2 = 22;
+        int actualOutput2 = solution.evalRPN(input2);
+        System.out.println("Test 2 passed? " + (expectedOutput2 == actualOutput2));
+    }
+}