Merge pull request TheAlgorithms#121 from manmeet-22/patch-3

Postfix Notation using Stack

Author: varunu28

Misc/StackPostfixNotation.java

@@ -0,0 +1,38 @@
+import java.util.*;
+
+public class Postfix {
+    public static void main(String[] args) {
+         Scanner scanner = new Scanner(System.in);
+        String post = scanner.nextLine();   // Takes input with spaces in between eg. "1 21 +"
+        System.out.println(postfixEvaluate(post));
+    }
+
+    // Evaluates the given postfix expression string and returns the result.
+    public static int postfixEvaluate(String exp) {
+	 	Stack<Integer> s = new Stack<Integer> ();
+		Scanner tokens = new Scanner(exp);
+
+		while (tokens.hasNext()) {
+			if (tokens.hasNextInt()) {
+				s.push(tokens.nextInt()); // If int then push to stack
+			} else {    // else pop top two values and perform the operation
+				int num2 = s.pop();
+				int num1 = s.pop();
+				String op = tokens.next();
+
+				if (op.equals("+")) {
+					s.push(num1 + num2);
+				} else if (op.equals("-")) {
+					s.push(num1 - num2);
+				} else if (op.equals("*")) {
+					s.push(num1 * num2);
+				} else {
+					s.push(num1 / num2);
+				}
+
+			//  "+", "-", "*", "/"
+			}
+		}
+		return s.pop();
+    }
+}