The ultimate guide to recursion interviews. Developed by FAANG engineers, practice with real-world C++ problems, interactive code playgrounds, and get interview-ready in just a few hours.

If you’ve ever struggled with solving coding problems using recursion, or if you’ve just got an interview coming up and want to brush up on your knowledge, you’ll definitely find this course helpful.
 
You’ll start with the basics of what recursion is and why it’s important before diving into what it looks like in practice. You’ll see how recursion can help you solve a variety of different math, string, and data structure problems, using interactive code playgrounds you can execute directly in your browser. You’ll have access to detailed explanations and visualizations for each problem to help you along the way.  
 
By the time you’re done, you’ll be able to use what you’ve learned to solve complex real-world problems, and even advance more easily through your interviews at top tech companies.

Recursion for Coding Interviews in C++

# Direct Recursion
If a function calls itself, it's known as **direct recursion**. This results in a one-step recursive call: the function makes a recursive call inside its own function body.


void directRecursionFunction()
{
  // some code...

  directRecursionFunction();

  // some code...
}

Below is an example of a direct recursive function that computes the square of a number:

#include <iostream>
using namespace std;

// recursive function to calculate square of a number
int square(int x)
 {
  // base case
    if (x == 0)
    {
      return x; 
    }

  // recursive case
  else 
   {
      return square(x-1) + (2*x) - 1;
   }
}
int main() {
  // implementation of square function
  int input=30;
  cout << input<<"^2 = "<<square(input);
  return 0;
}

# Indirect Recursion 
If the function f1 calls another function f2 and f2 calls f1 then it is **indirect recursion** (or mutual recursion). This is a two-step recursive call: the function calls another function to make a recursive call.
  


void indirectRecursionFunctionf1();
void indirectRecursionFunctionf2();
  
void indirectRecursionFunctionf1()
{
  // some code...

  indirectRecursionFunctionf2();

  // some code...
}

void indirectRecursionFunctionf2()
{
  // some code...

  indirectRecursionFunctionf1();

  // some code...
}

>**Note:** For indirect recursion, both the functions need to be declared **before** they are defined.

## What is an example of indirect recursion?
Below is an implementation of indirect recursion that prints the first 20 integers:

#include <iostream>
using namespace std;
int n=0;
// declaring functions
void foo1(void);
void foo2(void);

// defining recursive functions
void foo1() 
{ 
  if (n <= 20) 
  { 
    cout<<n<<" ";  // prints n
    n++;           // increments n by 1
    foo2();       // calls foo2() 
  } 
  else
    return; 
} 

void foo2() 
{ 
  if (n <= 20) 
  { 
    cout<<n<<" ";  // prints n
    n++;           // increments n by 1
    foo1();       // calls foo1()
  } 
  else
    return; 
} 

// Driver Program 
int main(void) 
{ 
  foo1(); 
  return 0; 
} 

This lesson explains two different types of recursion: direct and indirect recursion.

Recursion Fundamentals

Recursion with Numbers

Recursion With Strings

Recursion With Arrays

Recursion with Data Structures

Direct vs. Indirect Recursion