Added postfix expression evaluation problem

README.md

@@ -23,6 +23,8 @@ Collection of interview questions with Unit Tests. Problems includes Data Struct
   - [Baseball Game](src/_DataStructures_/Stack/baseball-game)
   - [Minimum Stack](src/_DataStructures_/Stack/min-stack)
   - [Balanced Parenthesis](src/_DataStructures_/Stack/balanced-parenthesis)
+  - [Postfix Expression Evaluation](src/_DataStructures_/Stack/postfix-expression-evaluation)
+
 
 - [Queue](src/_DataStructures_/Queue)
   - [Weave](src/_DataStructures_/Queue/weave)

src/_DataStructures_/Stack/postfix-expression-evaluation/index.js

@@ -0,0 +1,40 @@
+/**
+ * Evaluation of Postfix Expression
+ * Input:456*+
+ * Output:34
+ */
+
+const Stack = require('../index');
+
+function evaluatePostfixExpression(expression) {
+    let s = new Stack();
+    for (let i = 0; i < expression.length; i++) {
+        const char = expression[i];
+        if (!isNaN(char)) {
+            //if number push the char onto stack
+            s.push(Number(char));
+        } else {
+            // if char is an operator then pop two elements from stack, evaluate them accordingly based on operator.
+            //push the result to stack 
+            let val1 = s.pop();
+            let val2 = s.pop()
+            switch (char) {
+                case '+':
+                    s.push(val2 + val1);
+                    break;
+                case '-':
+                    s.push(val2 - val1);
+                    break;
+                case '*':
+                    s.push(val2 * val1);
+                    break;
+                case '/':
+                    s.push(val2 / val1);
+                    break;
+
+            }
+        }
+    }
+    //pop the value of postfix expression
+    return s.pop();
+}