Queue two stacks

codereview/twoStackQueue.md

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+## Purpose
+I recently came across this interview question
+> Implement a queue with 2 `Stack`s. Your queue should have an `enqueue` and a `dequeue` function.
+Assume you already have a `Stack` implementation
+
+## Discussion
+The way I thought about this problem is that one `Stack` would keep track of the input elements (i.e. what's been enqueued)
+and at some point, this `Stack` would transfer its elements to the second `Stack`.
+
+By `pop`ping the results of the first `Stack` onto the the second `Stack`, the second `Stack` now represents the elements, ordered
+by when they were first inputted into the queue.
+
+By `pop`ping off this second `Stack`, elements are now dequeued in the order that they were enqueued.
+
+What happens in the case where some elements are enqueued, an element is dequeued, and then a bunch of elements are enqueued?
+When does the transferring of elements from the first `Stack` to the second take place?
+
+In order to preserve the order of elements, the transfer of elements doesn't take place until the second `Stack` is empty.
+Thus, when dequeueing, first check that the second `Stack` is empty. If it isn't, then transfer the elements from the first `Stack`
+to the second.
+
+For example, take the case where `1`, `2`, and `3` are enqueued.
+1. The first `Stack` looks like this: `| 1 | 2 | 3|`.
+2. The second `Stack` is empty.
+3. When `dequeue` is called, `3`, `2`, and `1` are popped off the first `Stack` in that order.
+4. Now the first `Stack` is empty
+5. The second `Stack` looks like `| 3 | 2 | 1 |`.
+6. Since `dequeue` was called, we `pop` the second `Stack` (bye bye `1`) and now it looks like `| 3 | 2 |`.
+7. Now suppose `4` is enqueued.
+8. The first `Stack` is no longer empty - it looks like: `| 4 |`.
+9. Now suppose `dequeue` is called again - we can't immediately transfer `4` from the first `Stack` to the second since
+   the second `Stack` would look like `| 3 | 2 | 4 |`.
+10. Thus, we have to wait until all elements have been popped off the second `Stack` to transfer the elements from the first `Stack`.
+
+
+## Implementation
+
+<!-- language: lang-java --!>
+
+    import java.util.NoSuchElementException;
+    import java.util.Stack;
+
+    public class TwoStackQueue<T> {
+        private final Stack<T> input = new Stack<>();
+        private final Stack<T> output = new Stack<>();
+
+        public void enqueue(T data) {
+            input.add(data);
+        }
+
+        public T dequeue() {
+            if (isEmpty()) {
+                throw new NoSuchElementException("Unable to dequeue on empty queue");
+            }
+
+            transfer();
+
+            return output.pop();
+        }
+
+        public boolean isEmpty() {
+            return input.empty() && output.empty();
+        }
+
+        public T peek() {
+            if (isEmpty()) {
+                throw new NoSuchElementException("Unable to peek on empty queue");
+            }
+
+            transfer();
+
+            return output.peek();
+        }
+
+        private void transfer() {
+            if (output.empty()) {
+                while (!input.empty()) {
+                    output.push(input.pop());
+                }
+            }
+        }
+    }

src/main/java/problems/impl/TwoStackQueue.java

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+package problems.impl;
+
+import java.util.NoSuchElementException;
+import java.util.Stack;
+
+public class TwoStackQueue<T> {
+    private final Stack<T> input = new Stack<>();
+    private final Stack<T> output = new Stack<>();
+
+    public void enqueue(T data) {
+        input.add(data);
+    }
+
+    public T dequeue() {
+        if (isEmpty()) {
+            throw new NoSuchElementException("Unable to dequeue on empty queue");
+        }
+
+        transfer();
+
+        return output.pop();
+    }
+
+    public boolean isEmpty() {
+        return input.empty() && output.empty();
+    }
+
+    public T peek() {
+        if (isEmpty()) {
+            throw new NoSuchElementException("Unable to peek on empty queue");
+        }
+
+        transfer();
+
+        return output.peek();
+    }
+
+    private void transfer() {
+        if (output.empty()) {
+            while (!input.empty()) {
+                output.push(input.pop());
+            }
+        }
+    }
+}

src/test/java/problems/impl/TwoStackQueueTest.java

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+package problems.impl;
+
+import org.junit.Test;
+
+import java.util.NoSuchElementException;
+
+import static org.junit.Assert.*;
+
+public class TwoStackQueueTest {
+
+    private final String firstValue = "firstValue";
+    private final String secondValue = "secondValue";
+    private final String thirdValue = "thirdValue";
+
+    @Test
+    public void itShouldEnqueueElement() {
+        TwoStackQueue<String> queue = new TwoStackQueue<>();
+        queue.enqueue(firstValue);
+        assertFalse(queue.isEmpty());
+    }
+
+    @Test
+    public void itShouldEnqueueAndDequeueElements() {
+        TwoStackQueue<String> queue = new TwoStackQueue<>();
+        queue.enqueue(firstValue);
+        queue.enqueue(secondValue);
+        assertEquals(firstValue, queue.dequeue());
+        assertEquals(secondValue, queue.dequeue());
+    }
+
+    @Test
+    public void itShouldEnqueueAndDequeueElementsTwice() {
+        TwoStackQueue<String> queue = new TwoStackQueue<>();
+        queue.enqueue(firstValue);
+        queue.enqueue(secondValue);
+        assertEquals(firstValue, queue.dequeue());
+
+        queue.enqueue(thirdValue);
+        assertEquals(secondValue, queue.dequeue());
+        assertEquals(thirdValue, queue.dequeue());
+    }
+
+    @Test
+    public void itShouldThrowWhenDequeueingEmptyQueue() {
+        TwoStackQueue<String> queue = new TwoStackQueue<>();
+        try {
+            queue.dequeue();
+        } catch (NoSuchElementException e) {
+            // expected
+        }
+    }
+
+    @Test
+    public void itShouldEnqueueAndPeekElements() {
+        TwoStackQueue<String> queue = new TwoStackQueue<>();
+        queue.enqueue(firstValue);
+        queue.enqueue(secondValue);
+        assertEquals(firstValue, queue.peek());
+
+        queue.dequeue();
+        assertEquals(secondValue, queue.peek());
+    }
+
+    @Test
+    public void itShouldEnqueueAndPeekElementsTwice() {
+        TwoStackQueue<String> queue = new TwoStackQueue<>();
+        queue.enqueue(firstValue);
+        queue.enqueue(secondValue);
+        assertEquals(firstValue, queue.peek());
+
+        queue.dequeue();
+
+        queue.enqueue(thirdValue);
+        assertEquals(secondValue, queue.peek());
+
+        queue.dequeue();
+        assertEquals(thirdValue, queue.peek());
+    }
+
+    @Test
+    public void itShouldThrowWhenPeekingEmptyQueue() {
+        TwoStackQueue<String> queue = new TwoStackQueue<>();
+        try {
+            queue.peek();
+        } catch (NoSuchElementException e) {
+            // expected
+        }
+    }
+
+    @Test
+    public void itShouldBeEmptyForInstantiatedQueue() {
+        TwoStackQueue<String> queue = new TwoStackQueue<>();
+        assertTrue(queue.isEmpty());
+    }
+
+    @Test
+    public void itShouldBeEmptyAfterEnqueueingAndDequeueing() {
+        TwoStackQueue<String> queue = new TwoStackQueue<>();
+        queue.enqueue(firstValue);
+        assertFalse(queue.isEmpty());
+
+        queue.dequeue();
+        assertTrue(queue.isEmpty());
+    }
+}