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++

#include <iostream>
#include <string>
using namespace std; 

int totalVowels(string text, int len, int index) 
{ 
  //Write your code here
  int count=0;
  if (len==0){return 0;}
  char single=toupper(text[index]);
  if(single=='A' || single=='E' || single=='I' || single=='O' || single=='U')  
  {  
    count++;
  }
    return count + totalVowels(text,len-1,index+1);
} 


//Function to test your code
int main(){

  cout<<"The string is: Hello World"<<endl;
  cout<<"The total number of vowels in this string are: "<<totalVowels("Hello World",10,0)<<endl;

  cout<<"The string is: STR"<<endl;
  cout<<"The total number of vowels in this string are: "<<totalVowels("STR",3,0)<<endl;

  cout<<"The string is: AEIOUaeiouSs"<<endl;
  cout<<"The total number of vowels in this string are: "<<totalVowels("AEIOUaeiouSs",12,0)<<endl;

}




## Understanding the Code

Every recursive code has two functions; the recursive function and the main function.

### Driver Function

The recursive function is called within the driver function so lets first look at what it does, from **line 20 to 31**. 

* There are three calls made to the recursive function and for each call, it prints the given string and the total number of vowels in it. 

### Recursive Function

Now let's look at the recursive function which is the `totalVowels` function. Within a recursive function you have the **base case** and the **recursive case**. 

#### Base Case

* The base case in this function is the `if` condition, on *line 9*. This if condition simply states that if the `len` - the length of the input `text` string is equal to **0** then return the function with int **0**, as no vowels can be found in an empty string. 

#### Recursive case

* Provided that the length of the `text` string is not yet **0**, we move to the recursive case. The first step in the function is to create a `count` variable, set to **0** which will be incremented each time a vowel is found. 

* To keep uniformity, we take the element of `text` at position `index` and convert it to **uppercase**. This reduces the different checks we will have in the subsequent if conditions. Then we equate the changed element to a variable, `single` to keep the code **clean!**

* The *if condition* checks if `single` is equal to any of the vowels. If it is, then it increments `count` by 1. 

* After that the `return` statement simply calls on the recursive function again, `totalVowels`, adding the result of the subsequent recursive call to the last value of `count`. Note that this time, the `len` is reduced by **1** and the `index` is incremented by **1**. The former ensures that, at some point, our **base case** will be reached, and the latter ensures that the subsequent elements will be checked. 




# Understanding the Code

Every recursive code has two functions; the recursive function and the main function.

## Driver Function

The recursive function is called within the driver function so lets first look at what it does, from **line 20 to 31**. 

* There are three calls made to the recursive function and for each call, it prints the given string and the total number of vowels in it. 

## Recursive Function

Now let's look at the recursive function which is the `totalVowels` function. Within a recursive function you have the **base case** and the **recursive case**. 

### Base Case

* The base case in this function is the `if` condition, on *line 9*. This if condition simply states that if the `len` - the length of the input `text` string is equal to **0** then return the function with int **0**, as no vowels can be found in an empty string. 

### Recursive case

* Provided that the length of the `text` string is not yet **0**, we move to the recursive case. The first step in the function is to create a `count` variable, set to **0** which will be incremented each time a vowel is found. 

* To keep uniformity, we take the element of `text` at position `index` and convert it to **uppercase**. This reduces the different checks we will have in the subsequent if conditions. Then we equate the changed element to a variable, `single` to keep the code **clean!**

* The *if condition* checks if `single` is equal to any of the vowels. If it is, then it increments `count` by 1. 

* After that the `return` statement simply calls on the recursive function again, `totalVowels`, adding the result of the subsequent recursive call to the last value of `count`. Note that this time, the `len` is reduced by **1** and the `index` is incremented by **1**. The former ensures that, at some point, our **base case** will be reached, and the latter ensures that the subsequent elements will be checked. 




This lesson provides a detailed review of the solution to the challenge in the previous lesson.

Recursion Fundamentals

Recursion with Numbers

Recursion With Strings

Recursion With Arrays

Recursion with Data Structures

Solution Review 1: Let's Find the Total Number of Vowels

Solution: How many vowels were there?

Understanding the Code

Driver Function