Important Notice:

Practice-It will be discontinued as of November 1st, 2024. After this date, the website will remain online for a transitional period, but login will be restricted to University of Washington NetID authentication. This marks the next phase towards the platform's full retirement. Thank you for your use and support of the application over the years.

If you are looking for an alternative, a similar tool, CodeStepByStep, was developed independently by the original author of Practice-It, and is available at codestepbystep.com**

logo Practice-It logo

Main Page → Problems → Solve a Problem

compressDuplicates

Language/Type: Java Collections Stacks and Queues
Author: Stuart Reges (on 2014/02/13)

Write a method compressDuplicates that takes a stack of integers as a parameter and that replaces each sequence of duplicates with a pair of values representing a count of the number of duplicates followed by the number. For example, suppose a variable called s stores the following sequence of values: 

    bottom [2, 2, 2, 2, 2, -5, -5, 3, 3, 3, 3, 4, 4, 1, 0, 17, 17] top

and we make the following call: 

    compressDuplicates(s);

Then s should store the following values after the call: 

    bottom [5, 2, 2, -5, 4, 3, 2, 4, 1, 1, 1, 0, 2, 17] top

This new stack indicates that the original had 5 occurrences of 2 at the bottom of the stack followed by 2 occurrences of -5 followed by 4 occurrences of 3, and so on. This process works best when there are many duplicates in a row. For example, if the stack instead had stored: 

    bottom [10, 20, 10, 20, 10, 20] top

Then the resulting stack ends up being longer than the original: 

    bottom [1, 10, 1, 20, 1, 10, 1, 20, 1, 10, 1, 20] top

If the stack is empty, your method should not change it. You are to use one queue as auxiliary storage to solve this problem. You may not use any other auxiliary data structures to solve this problem, although you can have as many simple variables as you like. You may not use recursion to solve this problem and your solution must run in O(n) time. Use the Stack and Queue interfaces and the ArrayStack and LinkedQueue implementations discussed in lecture.

Type your solution here:


This is a method problem. Write a Java method as described. Do not write a complete program or class; just the method(s) above.

You must log in before you can solve this problem.


Log In

If you do not understand how to solve a problem or why your solution doesn't work, please contact your TA or instructor.
If something seems wrong with the site (errors, slow performance, incorrect problems/tests, etc.), please

Is there a problem? Contact a site administrator.