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 boolean isPalindrome(String text) {
        if (text.length() <= 1) {
            return true;
        }
        else {
            if (text.charAt(0) == text.charAt(text.length()-1)) {
                return isPalindrome(text.substring(1, text.length()-1));
            }
        }
        return false;
    }

    public static void main( String args[] ) { 
        String input1 = "hello";
        String input2 = "dadad";
        boolean answer1 = isPalindrome(input1);
        boolean answer2 = isPalindrome(input2);
        System.out.println("Is " + input1 + " a Palindrome? = " + answer1);
        System.out.println("Is " + input2 + " a Palindrome? = " + answer2);
    }  
}

## 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 19 to 24**. 

* In this driver method, we have two strings where `input2` is a palindrome and `input1` is not. 

* For each string, it calls on the `isPalindrome` method, and the output of the method is stored in the `answer1` and `answer2` variables. 

* On **line 23** and **line 24**, the code prints `true` if the string is a palindrome. If it is not a palindrome, the code prints `false`.


### Recursive Method

Now let’s look at the recursive method: the `isPalindrome` method from **line 4 to line 15**. Within this recursive method, you have the base case and the recursive case.

The method itself returns a `boolean` value, either **true** or **false** - depending on whether or not a palindrome is found. The method takes in one argument, the string `text` which is the string that will be checked. 

#### Base Case

* The first *if condition* from **line 4 to line 6** states that if the length of the given `text` is zero, it will return **false** indicating that the string is *not* a palindrome since it is an empty string.

* The *else if condition* from **line 7 to line 9** states that if a given `text` has a length equal to one, it will return **true**, since a string of a single letter string is technically a palindrome.



#### Recursive Case

Provided that the above-mentioned base conditions are not met, we move to the recursive case. 

* The *if condition* on **line 11** examines if the first and last index is equal. The function `isPalindrome` is called again-but with changed parameters. This enables the function to check if the entire string is a palindrome, rather than the first and last index. 

* We then pass the modified string `text` to the recursive method, `isPalindrome` comparing values from the next index using `substring(1, text.length()-1)` from the start of the string while decrementing the last index by 1.

* The recursive call, in this case, terminates when the values mentioned above become equal, or when the values do not match at their respective positions.

# 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 19 to 24**. 

* In this driver method, we have two strings where `input2` is a palindrome and `input1` is not. 

* For each string, it calls on the `isPalindrome` method, and the output of the method is stored in the `answer1` and `answer2` variables. 

* On **line 23** and **line 24**, the code prints `true` if the string is a palindrome. If it is not a palindrome, the code prints `false`.


## Recursive Method

Now let’s look at the recursive method: the `isPalindrome` method from **line 4 to line 15**. Within this recursive method, you have the base case and the recursive case.

The method itself returns a `boolean` value, either **true** or **false** - depending on whether or not a palindrome is found. The method takes in one argument, the string `text` which is the string that will be checked. 

### Base Case

* The first *if condition* from **line 4 to line 6** states that if the length of the given `text` is zero, it will return **false** indicating that the string is *not* a palindrome since it is an empty string.

* The *else if condition* from **line 7 to line 9** states that if a given `text` has a length equal to one, it will return **true**, since a string of a single letter string is technically a palindrome.



### Recursive Case

Provided that the above-mentioned base conditions are not met, we move to the recursive case. 

* The *if condition* on **line 11** examines if the first and last index is equal. The function `isPalindrome` is called again-but with changed parameters. This enables the function to check if the entire string is a palindrome, rather than the first and last index. 

* We then pass the modified string `text` to the recursive method, `isPalindrome` comparing values from the next index using `substring(1, text.length()-1)` from the start of the string while decrementing the last index by 1.

* The recursive call, in this case, terminates when the values mentioned above become equal, or when the values do not match at their respective positions.

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: Is this String a Palindrome?

Solution: Palindrome or not?

Understanding the Code

Driver Method