Implemented Queue from stacks (CoffeelessProgrammer#4)

Author: CoffeelessProgrammer

Data-Structures/Sequential/Queue.ts

@@ -16,6 +16,10 @@ export default class Queue<T> {
     return this.length;
   }
 
+  public isEmpty(): boolean {
+    return this.length === 0;
+  }
+
   public peek(): T | null {
     return this.first?.getValue() || null;
   }

Data-Structures/Sequential/Stack.ts

@@ -11,6 +11,10 @@ export default class Stack<T> {
     return this.length;
   }
 
+  public isEmpty(): boolean {
+    return this.length === 0;
+  }
+
   public peek(): T | null {
     return this.values[this.length-1] || null;
   }

Data-Structures/Sequential/StackLL.ts

@@ -16,6 +16,10 @@ export default class StackLL<T> {
     return this.length;
   }
 
+  public isEmpty(): boolean {
+    return this.length === 0;
+  }
+
   public peek(): T | null {
     return this.top?.getValue() || null;
   }

Playground/Challenges/StackifiedQueue.ts

@@ -0,0 +1,125 @@
+import Stack from '../../Data-Structures/Sequential/Stack.ts';
+
+// Ahhhhh! What a glorious day to do things just because we can! ^.^
+
+class StackifiedQueue<T> {
+  private first: Stack<T>;    // Stack with elements ordered first-on-top
+  private last: Stack<T>;     // Stack with elements ordered last-on-top
+  private length: number;
+
+  constructor() {
+    this.first = new Stack();
+    this.last = new Stack();
+    this.length = 0;
+  }
+
+  public getlength(): number {
+    return this.length;
+  }
+
+  public isEmpty(): boolean {
+    return this.length === 0;
+  }
+
+  public enqueue(value: T): boolean {
+
+    // Transfer all items from first-on-top to last-on-top
+    this._populateLastOnTop();
+
+    // Add to end of line, i.e. push to last-on-top stack
+    this.last.push(value);
+    ++this.length;
+
+    return true;
+  }
+
+  public dequeue(): T | null {
+    if(this.length === 0) return null;
+
+    this._populateFirstOnTop();
+    --this.length;
+
+    return this.first.pop();
+  }
+
+  public peek(): T | null {
+    if (this.length === 0) return null;
+    this._populateFirstOnTop();
+    return this.first.peek();
+  }
+
+  private _populateFirstOnTop(): void {
+    if (this.first.getLength() > 0) return;
+
+    for (let i = 0; i < this.length; ++i) {
+      this.first.push(this.last.pop());
+    }
+  }
+
+  private _populateLastOnTop(): void {
+    if (this.last.getLength() > 0) return;
+
+    for (let i = 0; i < this.length; ++i) {
+      this.last.push(this.first.pop());
+    }
+  }
+}
+
+function printQueue(queue: StackifiedQueue<any>) {
+  console.log(JSON.stringify(queue));
+}
+
+function printPeekQueue(queue: StackifiedQueue<any>) {
+  console.log('Peeking... Found', JSON.stringify(queue.peek()));
+}
+
+function printDequeue(queue: StackifiedQueue<any>) {
+  console.log('Dequeued:', JSON.stringify(queue.dequeue()));
+}
+
+
+//---------------------------------------------------------------------
+// ----------                 MAIN PROGRAM                   ----------
+//---------------------------------------------------------------------
+if (import.meta.main) {
+
+  const ATLA = new StackifiedQueue<string>();
+
+  printPeekQueue(ATLA);
+  ATLA.enqueue('Sokka');
+  ATLA.enqueue('Katara');
+  printPeekQueue(ATLA);
+  ATLA.enqueue('Aang');
+  ATLA.enqueue('Appa');
+
+  printQueue(ATLA);
+
+  printDequeue(ATLA);
+  printDequeue(ATLA);
+  printDequeue(ATLA);
+
+  printQueue(ATLA);
+  
+  printDequeue(ATLA);
+
+  printQueue(ATLA);
+
+  ATLA.enqueue('Zuko');
+  ATLA.enqueue('Iroh');
+
+  printQueue(ATLA);
+
+  // RUN: deno run Playground/Challenges/StackifiedQueue.ts
+}
+
+// --------------------------- Terminal Output: ---------------------------
+// Peeking... Found null
+// Peeking... Found "Sokka"
+// {"first":{"length":0,"values":[null,null]},"last":{"length":4,"values":["Sokka","Katara","Aang","Appa"]},"length":4}
+// Dequeued: "Sokka"
+// Dequeued: "Katara"
+// Dequeued: "Aang"
+// {"first":{"length":1,"values":["Appa",null,null,null]},"last":{"length":0,"values":[null,null,null,null]},"length":1}
+// Dequeued: "Appa"
+// {"first":{"length":0,"values":[null,null,null,null]},"last":{"length":0,"values":[null,null,null,null]},"length":0}
+// {"first":{"length":0,"values":[null,null,null,null]},"last":{"length":2,"values":["Zuko","Iroh",null,null]},"length":2}