Chapter 4. Quicksort


In this chapter

  • You learn about divide-and-conquer. Sometimes you’ll come across a problem that can’t be solved by any algorithm you’ve learned. When a good algorithmist comes across such a problem, they don’t just give up. They have a toolbox full of techniques they use on the problem, trying to come up with a solution. Divide-and-conquer is the first general technique you learn.
  • You learn about quicksort, an elegant sorting algorithm that’s often used in practice. Quicksort uses divide-and-conquer.

You learned all about recursion in the last chapter. This chapter focuses on using your new skill to solve problems. We’ll explore divide and conquer (D&C), a well-known recursive technique for solving problems.

This chapter really gets into the meat of algorithms. After all, an algorithm isn’t very useful if it can only solve one type of problem. Instead, D&C gives you a new way to think about solving problems. D&C is another tool in your toolbox. When you get a new problem, you don’t have to be stumped. Instead, you can ask, “Can I solve this if I use divide and conquer?”

At the end of the chapter, you’ll learn your first major D&C algorithm: quicksort. Quicksort is a sorting algorithm, and a much faster one than selection sort (which you learned in chapter 2). It’s a good example of elegant code.

Divide & conquer



Big O notation revisited