The ultimate guide to recursion interviews in JavaScript. Developed by FAANG engineers. Practice with real-world interview questions and get interview-ready in just a few hours.

If you’ve ever struggled with solving problems using recursion, or if you have to brush up your skills for an interview, this course is for you.

We'll start with the basics of what recursion is and why it’s important before diving into practicing solving actual questions. You’ll have access to detailed explanations and visualizations for each problem to help you along the way. 

By the time you have completed this course, you’ll be able to use all the concepts to solve complex real-world problems. We hope that through practicing recursion, you will be able to ace all recursion related questions in interviews at top tech companies.

Recursion for Coding Interviews in JavaScript

## What does "Removing Adjacent Duplicates from a String" Mean?
This means that we'll remove all extra instances of a character when multiple instances are found together. In other words, only one instance should remain after this process. 

> Lower and upper case letters are considered different characters. Example: string `Hhelo` doesn't contain any duplicates.

Repeated occurrences of the same letter within a string are allowed as long as they are not next to each other. Example: string `hele` has no adjacent duplicates.

### Explanation
To remove duplicates, we reduce the length of the string with each recursive call. If the current character is similar to the following character, we discard its first occurrence and repeat the process recursively on the rest of the string. However, if the current character is not similar to the following character, we keep it. We append it when the child function returns.


The code snippet above checks for $3$ cases:
1. **(Line number 3 to 5)** If the length of the string is less than or equal to $1$.
   * There can be no duplicates in a string that contains only $1$ character or no characters so we return the string variable in this case.

> Condition $1$ is the **base case** since there is no string manipulation occurring in this case.

2. **(Line number 8 to 10)** If the first and second elements of the string are same.
   * In this case, we discard the first occurrence and recursively call the same function with the first letter removed.

3. **(Line number 13)** If none of these $2$ conditions are satisfied, we save the first element for later use and call the same function again with the first element removed. The saved element will be later appended to the beginning of the string when the recursive call returns.

Let's take a look at the sequence of function calls:

# What does "Removing Adjacent Duplicates from a String" Mean?
This means that we'll remove all extra instances of a character when multiple instances are found together. In other words, only one instance should remain after this process. 

> Lower and upper case letters are considered different characters. Example: string `Hhelo` doesn't contain any duplicates.

Repeated occurrences of the same letter within a string are allowed as long as they are not next to each other. Example: string `hele` has no adjacent duplicates.

## Explanation
To remove duplicates, we reduce the length of the string with each recursive call. If the current character is similar to the following character, we discard its first occurrence and repeat the process recursively on the rest of the string. However, if the current character is not similar to the following character, we keep it. We append it when the child function returns.


The code snippet above checks for $3$ cases:
1. **(Line number 3 to 5)** If the length of the string is less than or equal to $1$.
   * There can be no duplicates in a string that contains only $1$ character or no characters so we return the string variable in this case.

> Condition $1$ is the **base case** since there is no string manipulation occurring in this case.

2. **(Line number 8 to 10)** If the first and second elements of the string are same.
   * In this case, we discard the first occurrence and recursively call the same function with the first letter removed.

3. **(Line number 13)** If none of these $2$ conditions are satisfied, we save the first element for later use and call the same function again with the first element removed. The saved element will be later appended to the beginning of the string when the recursive call returns.

Let's take a look at the sequence of function calls:

In this lesson, we'll learn how to remove all adjacent duplicates from a string using recursion.

Recursion Fundamentals

Iteration Vs Recursion

Recursion with Numbers

Recursion with Strings

Recursion with Arrays

Recursion with Data Structures

Conclusion

Remove all Adjacent Duplicates from a String

What does “Removing Adjacent Duplicates from a String” Mean?

Implementation

Explanation