Coding Interview Question: Stack from Queues

March 4, 2016

Question

Implement a LIFO stack with basic functionality (push and pop) using FIFO queues to store the data.

Once you think that you’ve solved the problem, click below to see the solution.

As always, remember that practicing coding interview questions is as much about how you practice as the question itself. Make sure that you give the question a solid go before skipping to the solution. Ideally if you have time, write out the solution first by hand and then only type it into your computer to verify your work once you've verified it manually. To learn more about how to practice, check out this blog post.

Solution

How was that problem? You can check out the solution in the video below.

Here is the source code for the solution shown in the video:

public class Stack {

    private Queue<Integer> primary = new LinkedList<Integer>();
    private Queue<Integer> secondary = new LinkedList<Integer>();

    public Stack() {}

    public void push(int x) {
        secondary.add(x);
        while (!primary.isEmpty()) {
            secondary.add(primary.remove()); 
        }

        Queue<Integer> temp = primary;
        primary = secondary;
        secondary = temp;
    }

    public int pop() {
        if (primary.isEmpty()) throw new IndexOutOfBoundsException();
        return primary.remove();
    }
}

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public class Stack {

private Queue<Integer> primary = new LinkedList<Integer>();

private Queue<Integer> secondary = new LinkedList<Integer>();

public Stack() {}

public void push(int x) {

secondary.add(x);

while (!primary.isEmpty()) {

secondary.add(primary.remove());

}

Queue<Integer> temp = primary;

primary = secondary;

secondary = temp;

}

public int pop() {

if (primary.isEmpty()) throw new IndexOutOfBoundsException();

return primary.remove();

}

Did you get the right answer to this coding interview question? Please share your thoughts in the comments below.

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