Add QueueByTwoStacks algorithm (#5623)

src/main/java/com/thealgorithms/datastructures/queues/QueueByTwoStacks.java

@@ -0,0 +1,87 @@
+package com.thealgorithms.datastructures.queues;
+
+import java.util.NoSuchElementException;
+import java.util.Stack;
+
+/**
+ * A queue implementation using two stacks. This class provides methods to
+ * enqueue (add) elements to the end of the queue and dequeue (remove)
+ * elements from the front, while utilizing two internal stacks to manage
+ * the order of elements.
+ *
+ * @param <T> The type of elements held in this queue.
+ */
+public class QueueByTwoStacks<T> {
+
+    private final Stack<T> enqueueStk;
+    private final Stack<T> dequeueStk;
+
+    /**
+     * Constructor that initializes two empty stacks for the queue.
+     * The `enqueueStk` is used to push elements when enqueuing, and
+     * the `dequeueStk` is used to pop elements when dequeuing.
+     */
+    public QueueByTwoStacks() {
+        enqueueStk = new Stack<>();
+        dequeueStk = new Stack<>();
+    }
+
+    /**
+     * Adds an element to the end of the queue. This method pushes the element
+     * onto the `enqueueStk`.
+     *
+     * @param item The element to be added to the queue.
+     */
+    public void put(T item) {
+        enqueueStk.push(item);
+    }
+
+    /**
+     * Removes and returns the element at the front of the queue.
+     * If `dequeueStk` is empty, it transfers all elements from
+     * `enqueueStk` to `dequeueStk` to maintain the correct FIFO
+     * (First-In-First-Out) order before popping.
+     *
+     * @return The element at the front of the queue.
+     * @throws NoSuchElementException If the queue is empty.
+     */
+    public T get() {
+        if (dequeueStk.isEmpty()) {
+            while (!enqueueStk.isEmpty()) {
+                dequeueStk.push(enqueueStk.pop());
+            }
+        }
+        if (dequeueStk.isEmpty()) {
+            throw new NoSuchElementException("Queue is empty");
+        }
+        return dequeueStk.pop();
+    }
+
+    /**
+     * Returns the total number of elements currently in the queue.
+     * This is the sum of the sizes of both stacks.
+     *
+     * @return The number of elements in the queue.
+     */
+    public int size() {
+        return enqueueStk.size() + dequeueStk.size();
+    }
+
+    /**
+     * Returns a string representation of the queue, showing the elements
+     * in the correct order (from front to back).
+     * The `dequeueStk` is first cloned, and then all elements from the
+     * `enqueueStk` are added to the cloned stack in reverse order to
+     * represent the queue accurately.
+     *
+     * @return A string representation of the queue.
+     */
+    @Override
+    public String toString() {
+        Stack<T> tempStack = (Stack<T>) dequeueStk.clone();
+        while (!enqueueStk.isEmpty()) {
+            tempStack.push(enqueueStk.pop());
+        }
+        return "Queue(" + tempStack + ")";
+    }
+}