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 decimalToBinary(int decimalNum) {
        if (decimalNum == 0) {
            return 0;
        }
        else {
            return (decimalNum%2 + 10*decimalToBinary(decimalNum/2));
        }
    }

    public static void main( String args[] ) {
        int input = 27;
        int result = decimalToBinary(input);
        System.out.println("The binary form of " + input + " is: " + result);
    }

}

# Understanding the Code

In the code above, the method `decimalToBinary` is a recursive method, since it calls itself in the method body. Below is an explanation of the above code:

## Driver Method 
* In the `main` code, we have defined an integer variable, an `input` that represents the decimal number that is to be converted to its equivalent binary number.

* The method `decimalToBinary` is called on **line 14** and takes the `input` variable as its argument. 

* The `System.out.println` command on **line 15** prints the integer value that returned from the `decimalToBinary` method.

## Recursive Method
* The return type of this method is `int` because the binary number will always be in an integer form. 

* The `decimalToBinary` method takes in only one integer, `decimalNum`, as its input parameter.

* It consists of a **base case** and a **recursive case** both of which are explained in detail below.

### Base Case
* The base case makes sure that if or when the `decimalNum` equals to $0$, the method should return $0$ and get terminated.  

### Recursive Case
* If the base condition does not compute to be true, the method enters the `else` block at **line 8**, where the **recursive call** is made. Note that in this recursive call, the input parameter is `(decimalNum/2)`, which is multiplied by $10$, and `decimalNum%2` is further added to it.

* With each recursive call, the input parameter `decimalNum` is reduced by half.

Let us look at the example below to see why this input parameter is passed to the recursive function.

Let the decimal number be $9$.

1. $9$ $\%$ $2$ (which equals to $1$) $+$ $10$ $*$ $(9/2)$

2. $4$ $\%$ $2$ (which equals to $0$) $+$ $10$ $*$ $(4/2)$ 

3. $2$ $\%$ $2$ (which equals to $0$) $+$ $10$ $*$ $(2/2)$ 

4. $1$ $\%$ $2$ (which equals to $1$) $+$ $10$ $*$ $(1/2)$ 

The output equivalent binary number for $9$ is thus $1001$.

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: Convert Decimal Number to Binary Number

Solution: From Decimal to Binary

Understanding the Code

Driver Method