feat: implement queue using stack

13.Implement-a-Queue-by-using-Stack.md

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+# 13. Implement a Queue by using Stack
+
+### Problem
+
+https://bigfrontend.dev/problem/implement-a-queue-by-using-stack
+
+#
+
+### Problem Description
+
+In JavaScript, we could use array to work as both a Stack or a queue.
+
+```js
+const arr = [1, 2, 3, 4];
+
+arr.push(5); // now array is [1, 2, 3, 4, 5]
+arr.pop(); // 5, now the array is [1, 2, 3, 4]
+```
+
+Above code is a Stack, while below is a Queue
+
+```js
+const arr = [1, 2, 3, 4];
+
+arr.push(5); // now the array is [1, 2, 3, 4, 5]
+arr.shift(); // 1, now the array is [2, 3, 4, 5]
+```
+
+now suppose you have a stack, which has only follow interface:
+
+```js
+class Stack {
+  push(element) {
+    /* add element to stack */
+  }
+  peek() {
+    /* get the top element */
+  }
+  pop() {
+    /* remove the top element */
+  }
+  size() {
+    /* count of elements */
+  }
+}
+```
+
+Could you implement a Queue by using only above Stack? A Queue must have following interface
+
+```js
+class Queue {
+  enqueue(element) {
+    /* add element to queue, similar to Array.prototype.push */
+  }
+  peek() {
+    /* get the head element */
+  }
+  dequeue() {
+    /* remove the head element, similar to Array.prototype.pop */
+  }
+  size() {
+    /* count of elements */
+  }
+}
+```
+
+**notes**
+
+you can only use Stack as provided, Array should be avoided for the purpose of practicing.
+
+#
+
+### Solution 1 (O(1) enqueue, O(n) dequeue and peek)
+
+```js
+/* you can use this Class which is bundled together with your code
+
+class Stack {
+  push(element) { // add element to stack }
+  peek() { // get the top element }
+  pop() { // remove the top element}
+  size() { // count of element }
+}
+*/
+
+/* Array is disabled in your code */
+
+// you need to complete the following Class
+class Queue {
+  constructor() {
+    this.content = new Stack();
+  }
+
+  enqueue(element) {
+    this.content.push(element);
+  }
+  peek() {
+    const reversedQueue = this._reverse(this.content);
+    return reversedQueue.peek();
+  }
+  size() {
+    return this.content.size();
+  }
+  dequeue() {
+    const reversedQueue = this._reverse(this.content);
+    const poppedItem = reversedQueue.pop();
+    this.content = this._reverse(reversedQueue);
+    return poppedItem;
+  }
+  _reverse(queue) {
+    const reversedQueue = new Stack();
+    while (queue.size() > 0) {
+      const lastItem = queue.pop();
+      reversedQueue.push(lastItem);
+    }
+
+    return reversedQueue;
+  }
+}
+```
+
+#
+
+### Solution 2 (O(n) enqueue, O(1) dequeue and peek)
+
+```js
+/* you can use this Class which is bundled together with your code
+
+class Stack {
+  push(element) { // add element to stack }
+  peek() { // get the top element }
+  pop() { // remove the top element}
+  size() { // count of element }
+}
+*/
+
+/* Array is disabled in your code */
+
+// you need to complete the following Class
+class Queue {
+  constructor() {
+    this.content = new Stack();
+    this.reversedContent = new Stack();
+  }
+
+  enqueue(element) {
+    while (this.content.size() > 0) {
+      const poppedItem = this.content.pop();
+      this.reversedContent.push(poppedItem);
+    }
+
+    this.reversedContent.push(element);
+
+    while (this.reversedContent.size() > 0) {
+      const poppedItem = this.reversedContent.pop();
+      this.content.push(poppedItem);
+    }
+  }
+  peek() {
+    return this.content.peek();
+  }
+  size() {
+    return this.content.size();
+  }
+  dequeue() {
+    return this.content.pop();
+  }
+}
+```