A good answer might be:

You stop applying the hammer when each sub-problem (rock) is small enough.

Recursion on the Rocks

Here (again) are the two parts to recursion:

  1. If the problem is easy, solve it immediately.
  2. If the problem can't be solved immediately, divide it into smaller problems, then:
    • Solve the smaller problems by applying this procedure to each of them.

Here are how this applies to the rock-destruction problem:

  1. When a piece is small, don't pound it any further.
  2. To destroy a large rock, hit it with a hammer. The rock shatters, leaving small and large pieces.
    • Apply this procedure to each of the pieces.


Say that you wish to divide a line into 16 equal pieces.

What is the first thing that you do?