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import java.util.Stack;

/**

* This implements Queue using two Stacks.

*

* Big O Runtime:

* insert(): O(1)

* remove(): O(1) amortized

* isEmpty(): O(1)

*

* A queue data structure functions the same as a real world queue.

* The elements that are added first are the first to be removed.

* New elements are added to the back/rear of the queue.

*

* @author sahilb2

*

*/

class QueueWithStack {

// Stack to keep track of elements inserted into the queue

private Stack inStack;

// Stack to keep track of elements to be removed next in queue

private Stack outStack;

/**

* Constructor

*/

public QueueWithStack() {

this.inStack = new Stack();

this.outStack = new Stack();

}

/**

* Inserts an element at the rear of the queue

*

* @param x element to be added

*/

public void insert(Object x) {

// Insert element into inStack

this.inStack.push(x);

}

/**

* Remove an element from the front of the queue

*

* @return the new front of the queue

*/

public Object remove() {

if(this.outStack.isEmpty()) {

// Move all elements from inStack to outStack (preserving the order)

while(!this.inStack.isEmpty()) {

this.outStack.push( this.inStack.pop() );

}

}

return this.outStack.pop();

}

/**

* Peek at the element from the front of the queue

*

* @return the new front of the queue

*/

public Object peek() {

if(this.outStack.isEmpty()) {

// Move all elements from inStack to outStack (preserving the order)

while(!this.inStack.isEmpty()) {

this.outStack.push( this.inStack.pop() );

}

}

return this.outStack.peek();

}

/**

* Returns true if the queue is empty

*

* @return true if the queue is empty

*/

public boolean isEmpty() {

return (this.inStack.isEmpty() && this.outStack.isEmpty());

}

}

/**

* This class is the example for the Queue class

*

* @author sahilb2

*

*/

public class QueueUsingTwoStacks {

/**

* Main method

*

* @param args Command line arguments

*/

public static void main(String args[]){

QueueWithStack myQueue = new QueueWithStack();

myQueue.insert(1);

// instack: [(top) 1]

// outStack: []

myQueue.insert(2);

// instack: [(top) 2, 1]

// outStack: []

myQueue.insert(3);

// instack: [(top) 3, 2, 1]

// outStack: []

myQueue.insert(4);

// instack: [(top) 4, 3, 2, 1]

// outStack: []

System.out.println(myQueue.isEmpty()); //Will print false

System.out.println(myQueue.remove()); //Will print 1

// instack: []

// outStack: [(top) 2, 3, 4]

myQueue.insert(5);

System.out.println(myQueue.peek()); //Will print 2

// instack: [(top) 5]

// outStack: [(top) 2, 3, 4]

myQueue.remove();

System.out.println(myQueue.peek()); //Will print 3

// instack: [(top) 5]

// outStack: [(top) 3, 4]

myQueue.remove();

System.out.println(myQueue.peek()); //Will print 4

// instack: [(top) 5]

// outStack: [(top) 4]

myQueue.remove();

// instack: [(top) 5]

// outStack: []

System.out.println(myQueue.peek()); //Will print 5

// instack: []

// outStack: [(top) 5]

myQueue.remove();

// instack: []

// outStack: []

System.out.println(myQueue.isEmpty()); //Will print true

}

}