Dang Khoa - Topic 3: Evaluate Reverse Polish Notation

Topic3_StackQueue/EvaluateReversePolishNotation.java

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+import java.util.Stack;
+
+public class EvaluateRPN {
+    // Method to evaluate Reverse Polish Notation
+    // Time Complexity: O(N), where N is the number of tokens in the input array.
+    // Space Complexity: O(N),In the worst case, the stack may hold all the operands before applying any operators.
+    public int evalRPN(String[] tokens) {
+        Stack<Integer> stack = new Stack<>(); // Initialize a stack to hold operands
+
+        // Loop through each token in the input array
+        for (String token : tokens) {
+            switch (token) {
+                // If the token is '+', pop the top two elements, add them, and push the result
+                case "+":
+                    stack.push(stack.pop() + stack.pop());
+                    break;
+
+                // If the token is '-', pop the top two elements, subtract the second from the first, and push the result
+                case "-":
+                    int b = stack.pop();
+                    int a = stack.pop();
+                    stack.push(a - b);
+                    break;
+
+                // If the token is '*', pop the top two elements, multiply them, and push the result
+                case "*":
+                    stack.push(stack.pop() * stack.pop());
+                    break;
+
+                // If the token is '/', pop the top two elements, divide the first by the second, and push the result
+                case "/":
+                    b = stack.pop();
+                    a = stack.pop();
+                    stack.push(a / b);
+                    break;
+
+                // If the token is a number, parse it and push it onto the stack
+                default:
+                    stack.push(Integer.parseInt(token));
+            }
+        }
+
+        // The result of the RPN expression is the last remaining element in the stack
+        return stack.pop();
+    }
+
+    public static void main(String[] args) {
+        EvaluateRPN evaluator = new EvaluateRPN();
+        String[] tokens = {"2", "1", "+", "3", "*"};
+        System.out.println(evaluator.evalRPN(tokens));
+    }
+}