The ultimate recursion guide! Master basics, solve real-world problems, and get interview-ready in hours with FAANG engineers’ proven strategies.

If you’ve ever struggled with solving coding problems using recursion, or if you need to brush up your recursion skills for an interview, this course is for you! We will start with the basics of recursion before we practice solving actual coding problems. 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 of recursion to solve complex, real-world problems. We hope that this course will help you ace all the recursion related questions during interviews at top tech companies!

Recursion for Coding Interviews in Python

def firstIndex(arr, testVariable, currentIndex): # returns the first occurrence of testVariable
  while currentIndex < len(arr) : # Iterate over the array
    if arr[currentIndex] == testVariable :  # Return the current index if testVariable found
      return currentIndex
      
    currentIndex += 1
  return -1

# Driver Code
arr = [9, 8, 1, 8, 1, 7]
testVariable = 1
currentIndex = 0
print(firstIndex(arr, testVariable, currentIndex))


## Explanation
To find the first occurrence of `testVariable` we iterate over the entire array and return the `currentIndex` when `testVariable` is found. Let's look at an illustration:

def firstIndex(arr, testVariable, currentIndex) : # returns the first occurrence of testVariable
  # Base Case1
  if len(arr) == currentIndex :
    return -1;

  # Base Case2  
  if arr[currentIndex] == testVariable :
    return currentIndex

  # Recursive Case
  return firstIndex(arr, testVariable, currentIndex + 1)

# Driver Code
arr = [9, 8, 1, 8, 1, 7]
testVariable = 1
currentIndex = 0

print(firstIndex(arr, testVariable, currentIndex))

## Explanation
In the **iterative method**, we reduce the length of the array with each iteration.

In recursion, we follow the same method: we decrease the length of the array each time and pass it to another execution instance. This will be our **recursive case**.

For the **base case**, we have two conditions:
1. When the length of the array is equal to the `currentIndex`. This means that the entire array has been traversed recursively and the `testVariable` was not found. If it is not found, we return $-1$.
```python
    if len(arr) == currentIndex :
        return -1;
```
2. When `testVariable` is found at the `currentIndex`, In this case, it will return the `currentIndex`.
```python
    if arr[currentIndex] == testVariable :
        return currentIndex
```
The illustration below explains the solution in detail.

This review provides a detailed analysis for finding the first occurrence of a given number in an array.

Recursion Fundamentals

Iteration Vs. Recursion

Recursion with Numbers

Recursion with Strings

Recursion with Arrays

Recursion with Data Structures

Conclusion

Solution Review: Search First Occurrence of a Number

Solution #1: Iterative Method

Explanation

Solution #2: Recursive Method