Merge pull request #81 from knaxus/problems

Problems involving Queues

Author: TheSTL

README.md

@@ -30,10 +30,12 @@ Collection of interview questions with Unit Tests. Problems includes Data Struct
   - [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)
+  - [Reverse First K Elements of a Queue](src/_DataStructures_/Queue/reverse-first-k)
+  - [Generate all Binary Numbers from 1 to N](src/_DataStructures_/Queue/generate-binary-number)
+  - [Queue using Stack](src/_DataStructures_/Queue/queue-using-stack)
 
 - [Doubly Linked List](src/_DataStructures_/DoublyLinkedList)
 

src/_DataStructures_/Queue/generate-binary-number/index.js

@@ -0,0 +1,29 @@
+const Queue = require('../index');
+
+function generateBinaryNumber(n) {
+  const result = [];
+  const q = new Queue();
+
+  // add `1` to the queue
+  q.enqueue('1');
+
+  // iterate till the given number
+  for (let i = 0; i < n; i += 1) {
+    // push the item in the queue to the array
+    result.push(q.dequeue());
+
+    // append `0` & `1` respectively
+    const s1 = `${result[i]}0`;
+    const s2 = `${result[i]}1`;
+
+    // push the combinations in the queue
+    q.enqueue(s1);
+    q.enqueue(s2);
+  }
+  // return the result containing all the binary numbers
+  return result;
+}
+
+// console.log(generateBinaryNumber(5));
+
+module.exports = generateBinaryNumber;

src/_DataStructures_/Queue/queue-using-stack/index.js

@@ -0,0 +1,28 @@
+const Stack = require('../../Stack');
+
+class Queue {
+  constructor() {
+    this.queue = new Stack();
+    this.temp = new Stack();
+  }
+
+  enqueue(data) {
+    this.queue.push(data);
+  }
+
+  dequeue() {
+    if (!this.queue.peek()) {
+      return null;
+    }
+
+    // pop all the element to the temp stack
+    while (this.queue.peek()) this.temp.push(this.queue.pop());
+    const el = this.temp.pop();
+
+    // push all the temp items to the queue again
+    while (this.temp.peek()) this.queue.push(this.temp.pop());
+    return el;
+  }
+}
+
+module.exports = Queue;

src/_DataStructures_/Queue/reverse-first-k/index.js

@@ -0,0 +1,48 @@
+// eslint-disable-next-line no-unused-vars
+const Queue = require('../index');
+const Stack = require('../../Stack');
+
+function reverseFirstKElelments(q, k) {
+  const s = new Stack();
+
+  // push all the k elements ot the stack
+  for (let i = 0; i < k; i += 1) {
+    s.push(q.dequeue());
+  }
+
+  // push the stack items to the queue
+  for (let i = 0; i < k; i += 1) {
+    q.enqueue(s.pop());
+  }
+
+  // empty the queue and push the same queue
+  const remaining = q.length() - k;
+  for (let i = 0; i < remaining; i += 1) {
+    q.enqueue(q.dequeue());
+  }
+
+  // return the queue
+  return q;
+}
+
+module.exports = reverseFirstKElelments;
+
+// let q = new Queue();
+
+// q.enqueue(1);
+// q.enqueue(2);
+// q.enqueue(3);
+// q.enqueue(4);
+// q.enqueue(5);
+// q.enqueue(6);
+// q.enqueue(7);
+// q.enqueue(8);
+// q.enqueue(9);
+
+// q = reverseFirstKElelments(q, 4);
+
+// const arr = [];
+// while (q.length()) {
+//   arr.push(q.dequeue());
+// }
+// console.log(arr);

src/_DataStructures_/Stack/index.js

@@ -15,8 +15,10 @@ class Stack {
   peek() {
     return this.data[this.data.length - 1];
   }
-  isEmpty(){ //check if stack is empty
-    return this.data.length==0;
+
+  isEmpty() {
+    // check if stack is empty
+    return this.data.length === 0;
   }
 }