feat: add queue implementation using two stacks (#89)

Author: ajeshds

data_structures/stack_queue.ts

@@ -0,0 +1,92 @@
+/**
+ * A Stack Based Queue Implementation. 
+ * The Queue data structure which follows the FIFO (First in First Out) rule. 
+ * The dequeue operation in a normal stack based queue would be o(n), as the entire has to be shifted 
+ * With the help of two stacks, the time complexity of this can be brought down to amortized-O(1). 
+ * Here, one stack acts as an Enqueue stack where elements are added. 
+ * The other stack acts as a dequeue stack which helps in dequeuing the elements
+ */
+
+import { Queue } from './queue';
+import { Stack } from './stack';
+
+export class StackQueue<T> implements Queue<T>{
+
+    private enqueueStack: Stack<T> = new Stack<T>();
+    private dequeueStack: Stack<T> = new Stack<T>();
+
+    /**
+     * Returns the length of the Queue
+     *
+     * @returns {number} the length of the Queue
+     */
+    length(): number {
+        return this.enqueueStack.length() + this.dequeueStack.length();
+    }
+
+    /**
+     * Checks if the queue is empty.
+     *
+     * @returns {boolean} Whether the queue is empty or not.
+     */
+    isEmpty(): boolean {
+        return this.enqueueStack.isEmpty() && this.dequeueStack.isEmpty();
+    }
+
+    /**
+     * Adds an item to the queue.
+     * We always add a new item to the enqueueStack.
+     * @param item The item being added to the queue.
+     */
+    enqueue(item: T): void {
+        this.enqueueStack.push(item);
+    }
+
+    /**
+     * Shifts the elements from the enqueueStack to the dequeueStack
+     * In the worst case, all the elements from the enqueue stack needs to shifted, which needs O(n) time. 
+     * However, after the shift, elements can de dequeued at O(1).
+     * This helps in dequeuing the elements in amortized O(1) time.
+     */
+    private shift(): void {
+        while (!this.enqueueStack.isEmpty()) {
+            const enqueueStackTop = this.enqueueStack.pop();
+            this.dequeueStack.push(enqueueStackTop);
+        }
+    }
+
+    /**
+     * Removes an item from the queue and returns it.
+     *
+     * @throws Queue Underflow if the queue is empty.
+     * @returns The item that was removed from the queue.
+     */
+    dequeue(): T {
+        if (this.isEmpty()) {
+            throw new Error("Queue Underflow");
+        }
+
+        if (this.dequeueStack.isEmpty()) {
+            this.shift();
+        }
+
+        return this.dequeueStack.pop();
+    }
+
+    /**
+     * Returns the item at the front of the queue.
+     *
+     * @returns The item at the front of the queue or null if the queue is empty.
+     */
+    peek(): T | null {
+        if (this.isEmpty()) {
+            return null;
+        }
+
+        if (this.dequeueStack.isEmpty()) {
+            this.shift();
+        }
+
+        return this.dequeueStack.top();
+    }
+}

data_structures/test/stack_queue.test.ts

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+import { testQueue } from './queue';
+import { StackQueue } from '../stack_queue';
+
+describe("Stack Based Queue", () => testQueue(StackQueue));