Recursion is a technique where a function calls itself to solve smaller versions of a problem until it reaches a simple case it can solve directly. This simple case is called the base case and is important to stop the recursion from continuing forever.

Recursion is useful for problems like calculating factorial numbers or Fibonacci sequences, where the solution depends on smaller subproblems. Although recursion can make complex problems easier to understand and code, it uses more memory because each function call waits for the next one to finish. Ensure termination to avoid stack overflow.

int rec(int n) {
if (n == 0) return 1;
else return n + rec(n – 1);
}