What is Recursion ?

Recursion is the process in which a function calls itself directly or indirectly. And the function that calls itself is known as a recursive function.

How recursion works ?

Basic Structure of Recursion :

void recursion()
{
recursion();
}
int main()
{
recursion();
}

We all know that a C++ program terminates from main( ) function, similarly here main( ) function calls the recursion( ) function in very first step and then the recursion( ) function calls itself again. All kind of Recursion functions works in this similar way.

A Recursion Program :

//c++ program to calculate
//sum of natural numbers recursively

#include<iostream>
using namespace std;

int sum(int n)
{
if (n != 0)
return n + sum(n-1);
// sum() function calls itself
else
return n;
}

int main()
{
int num, result;

cout << "Enter a positive integer : ";
cin >> num;

result = sum(num);
//sum() function is called by main() function

cout << "Sum of first " << num << " numbers : " << result;

return 0;
}

Direct & Indirect Recursion :

Direct Recursion ~
A function is called direct recursive if it calls the same function itself.

void directFunction()
{
directFunction();
}
Indirect Recursion ~
A function is called indirect recursive function if it calls another and the called function calls the previous function directly or indirectly.

void indirectFunction1()
{
indirectFunction2();
}

void indirectFunction2()
{
indirectFunction1();
}