Added sort a stack problem and solution.

README.md

@@ -29,6 +29,8 @@ Collection of interview questions with Unit Tests. Problems includes Data Struct
   - [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)
+  - [Reverse First k Elements of Queue](src/_DataStructures_/Stack/reverse-first-k-elements-of-queue)
+
 
 - [Queue](src/_DataStructures_/Queue)
 

src/_DataStructures_/Queue/index.js

@@ -15,6 +15,15 @@ class Queue {
   remove() {
     return this.data.pop();
   }
+  isEmpty() {
+    return this.data.length == 0;
+  }
+  enqueue(element) {
+    this.data.push(element);
+  }
+  length() {
+    return this.data.length;
+  }
 }
 
-module.exports = Queue;
+module.exports = Queue;

src/_DataStructures_/Stack/reverse-first-k-elements-of-queue/index.js

@@ -0,0 +1,61 @@
+/**
+ * Reverse first K elements of a Queue
+ * Given an integer K and a queue of integers,reverse the order of the first K elements of the queue, leaving the other elements in the same relative order.
+ * Input : Q = [20, 19, 18, 17, 16, 15, 14, 13, 12, 11] k = 5
+ * Output : Q = [  20, 19, 18, 17, 16, 11, 12, 13, 14, 15]
+ * 
+ */
+//const Queue = require.resolve('Queue');
+
+const Queue = require('../../Queue/index');
+
+const Stack = require('../index');
+
+function reverseQueueElements(queue, k) {
+  let s = new Stack();
+
+  if (queue.isEmpty()) {
+    throw new Error("Empty Queue");
+  }
+  if(k<=0 || k>queue.length()){
+    throw new Error("Invalid K value");
+  }
+
+  for (let i = 0; i < k; i++) {
+    //push first k elements from queue and push in stack
+    s.push(queue.remove());
+  }
+
+  for (let i = 0; i < k; i++) {
+    //we reverse the k elements with the help of stack.
+    //pop all the k elements from stack and add to queue.
+    let no = s.pop();
+    queue.add(no);
+  }
+
+  for (let i = 0; i < queue.length() - k; i++) {
+    //bring the k elments again back to front to maintain the queue order.
+    queue.add(queue.peek());
+    queue.remove();
+
+  }
+  return queue;
+}
+
+/*
+let queue = new Queue();
+queue.add(11);
+queue.add(12);
+queue.add(13);
+queue.add(14);
+queue.add(15);
+queue.add(16);
+queue.add(17);
+queue.add(18);
+queue.add(19);
+queue.add(20);
+console.log("Queue before:", queue.data);
+let k = 5;
+let reversedQueue=reverseQueueElements(queue, k);
+console.log("Queue after:", queue.data);
+*/