The ultimate guide to recursion interviews with strategies developed by FAANG engineers. Practice with real-world interview questions, detailed explanations, and visualizations to prep faster.

If you’ve got an interview coming up and want to brush up on your knowledge, or if you’ve ever struggled with solving coding problems using recursion, you'll 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 by using interactive code playgrounds that 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 Java

class ChallengeClass {

    public static int totalVowels(String text, int len, int index) {
        int count = 0;

        if (len == 0) {
            return 0;
        }
        char single = Character.toUpperCase(text.charAt(index));
        if (single == 'A' || single == 'E' || single == 'I' || single == 'O' || single == 'U') {
            count++;
        }
        return count + totalVowels(text, len-1, index+1);
    }

    public static int callTotalVowels(String text) {
        return totalVowels(text, text.length(), 0);
    }

    public static void main( String args[] ) {
        String string1 = "Hello world";
        String string2 = "STR";
        String string3 = "AEIOUaeiouSs";

        int result1 = callTotalVowels(string1);
        int result2 = callTotalVowels(string2);
        int result3 = callTotalVowels(string3);

        System.out.println( "Total number of vowels in " + string1 + " are = " + result1);
        System.out.println( "Total number of vowels in " + string2 + " are = " + result2);
        System.out.println( "Total number of vowels in " + string3 + " are = " + result3);
    }
}

# Understanding the Code

Every recursive code has two methods; the recursive method and the main method.

## Driver Method

The recursive method is called within the driver method. Let's first look at what it does-from **line 21 to line 31**. 

* There are three calls made to the recursive method. Each call, prints the given string and the total number of vowels within it. 

## Recursive Method

Let's look at the recursive method,-which is the `totalVowels` method. Within this recursive method, you have the **base case** and the **recursive case**. 

### Base Case

* The base case in this method is the `if` condition, as shown on-**line 6**. This if condition simply states that if the `len` - the length of the input `text` string is equal to **0**, it will return the method with int **0**, since 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 method is to create a `count` variable that is set to **0**. This count variable will be incremented each time a vowel is found. 

* To keep uniformity, on **line 9**, 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.We equate the changed element within a variable, `single` to keep the code **uniform**

* The *if condition* from **line 10 to line 12** checks if `single` is equal to any of the vowels. If this is found to be true, it increments `count` by 1. 

* The `return` statement on **line 13** then calls on the recursive method, `totalVowels`, adding the result of the subsequent recursive call to the last value of `count`. Note that, the `len` is reduced by **1** and the `index` is incremented by **1**. The reduction by 1  ensures that our **base case** will be reached, and the increase of 1 ensures that the subsequent elements will be checked. 




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

Fundamentals of Recursion

Iteration to Recursion

Recursion with Numbers

Recursion with Strings

Recursion with Arrays

Recursion with Data Structures

Solution Review: Let's find the total number of vowels!

Solution: How many vowels are there?

Understanding the Code