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;

bool isPalindrome(string text) {
        if (text.size() <= 1) {
            return true;
        }
        else {
            if (text[0] == text[text.size()-1]) {
                return isPalindrome(text.substr(1, text.size()-2));
            }
        }
        return false;
    }

int main() { 
        string input1 = "toot";
        string input2 = "dadad";
        bool answer1 = isPalindrome(input1);
        bool answer2 = isPalindrome(input2);
        cout << "Is " << input1 << " a Palindrome? = " << answer1 << endl;
        cout << "Is " << input2 << " a Palindrome? = " << answer2 << 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 17 to 28**. 

* In this driver method we have an array of strings which has two palindrome and one non-palindrome text string. 

* For each value, it calls on the `isPalindrome` method and the output of the function is stored in an `answer` variable. 

* This is checked in an *if-condition*. Given the value is **0** for `answer`, **no** is displayed on the screen, otherwise a **yes** is displayed. 

## Recursive Function

Now let’s look at the recursive function which is the `isPalindrome` function. Within a recursive function you have the base case and the recursive case.

The method itself returns an integer, **1** or **0** - depending on whether a palindrome is found or not. The function takes in three arguments: the string `text` which is the string to be checked, the `front` which is the starting index, and the `back` which is the index of the last element in the given string.  

### Base Case

* There are three base cases within this function. 
  * The first case is on **line 7** which states that if the length of the given `text` is zero, then it returns **0** indicating that the string is *not* a palindrome as it is empty.

  * The second case is on **line 9** which states that given `text` has a length equal to one, simply return **1** as a single letter string is technically a palindrome.

  * The third base case, on **line 11** states that given the starting index, `front`, and the ending index, `back`, do not have the **same value**, it returns **0** as it cannot be a palindrome.

### Recursive Case

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

* On **line 13** the function is called again, with changed parameters in order to check if the entire string is a palindrome, not just the first and last index. 
* The string `text` is passed without any changes.
* We increment the `front` index by one, so that the next index can be checked.
* We decrement the `back` index by one, as this ensures that the correct corresponding index is being compared to the new `front` index.
* The recursive call in this case terminates when `front` and `back` either become equal, or values at their respective positions don't match. 

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 2: Is this String a Palindrome?

Solution: Palindrome or not?

Understanding the Code

Driver Function