Added postfix expression evaluation problem

README.md

@@ -26,7 +26,10 @@ 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)
+  - [Remove Consecutive Repeated Digits](src/_DataStructures_/Stack/remove-consecutive-repeated-digits)
   - [Implement 2 Stacks using Single Array](src/_DataStructures_/Stack/2-stacks-using1-array)
+
 
 - [Queue](src/_DataStructures_/Queue)
   - [Weave](src/_DataStructures_/Queue/weave)

src/_Classics_/fibonacci/index.js

@@ -1,9 +1,11 @@
+//The Fibonacci sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21
 // the algorithm has time complexity of O(n^2), very bad!
 function fibonacci(position) {
   // if position is 1 or 2, the number in fibonacci sequence will be 1
-  if (position < 3) {
-    return 1;
+  if (position <= 1) {
+    return position;
   }
+
   // else the element in fibonacci sequence will be the sum of
   // element at position(p) (p -1) and (p - 2)
   return fibonacci(position - 2) + fibonacci(position - 1);
@@ -24,8 +26,8 @@ function fibonacciMemoized(index, cache) {
   if (cache[index]) {
     return cache[index];
   } else {
-    if (index < 3) {
-      return 1;
+    if (index <=1) {
+      return index;
     } else {
       cache[index] =
         fibonacciMemoized(index - 1, cache) +
@@ -41,7 +43,9 @@ function fibonacciMemoized(index, cache) {
 
 function fibonacciTabular(n) {
   const table = [0, 1];
-
+  if (n <= 1) {
+    return n;
+  }
   for (let i = 2; i <= n; i += 1) {
     table[i] = table[i - 1] + table[i - 2];
   }
@@ -54,4 +58,4 @@ function fibonacciTabular(n) {
 // console.log(`Fib normal - ${fibonacci(number)}`);
 // console.log('--');
 // console.log(`Fib memo - ${fibonacciMemoized(number)}`);
-// console.log(`Fib table - ${fibonacciTabular(number)}`);
+// console.log(`Fib table - ${fibonacciTabular(number)}`);

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();
+}

src/_DataStructures_/Stack/remove-consecutive-repeated-digits/index.js

@@ -0,0 +1,25 @@
+/**
+ * Given an integer N, remove consecutive repeated digits from it.
+ * Input:133445
+ * Output:1345
+ */
+
+const Stack = require('../index');
+
+
+function removeConsecutiveDigits(no) {
+    let s = new Stack();
+    let newNo = "";
+    //initally push first digit into stack
+    newNo += no[0];
+    s.push(no[0]);
+    for (let i = 1; i < no.length; i++) {
+        const digit = no[i];
+        //if stack top and incoming digit is same ignore it else append to newNo.
+        if (s.peek() !== digit) {
+            newNo += digit;
+            s.push(digit);
+        }
+    }
+    return newNo
+}