A good bank of recursion solutions in C/C++/Java/C#

One of the best ways to learn recursion is to get some experience in a functional programming language such as Haskell or Lisp or Scheme.

So finding recursive problems can be reduced to finding some problems and answers related to functional programming languages. Here's an example 99 lisp problems.

It really only takes 5 minutes to learn Scheme or Lisp so you can get started with examples right away for the test tomorrow you mentioned.

Another great way to learn recursion is to get some practice in mathematical proofs involving induction.

Key concepts relating to recursion:

With recursion you don't need to know how to solve the problem. You just need to know 2 things. 1) how to solve the smallest instance of the problem, and 2) how to break it up into smaller parts.

Equivalently, you just have to keep in mind that you need: 1) a base case and 2) a recursive case.

The base case handles 1 single instance of what you want to do with smallest input.

The recursive case breaks the problem into a subproblem. Eventually this subproblem will reduce to the base case.

Example:

//1+...+n = n*n(+1)/2 = sumAll(n):

int sumAll(int x)
{
  if(x == 0) //base case
    return 0; 
  else
    return sumAll(x-1) + x; //recursive case
}

It is important to understand that the base case is not hard to figure out. It just has to exist. Here is an equivalent solution for x> 0:

//1+...+n = n*n(+1)/2 = sumAll(n):
int sumAll(int x)
{
  if(x == 1) //base case
    return 1; 
  else
    return sumAll(x-1) + x; //recursive case
}