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

```python
  elif string[:len(b)] == b :
    return a + replace(string[len(b):], a, b)
    
  else :
    return string[0] + replace(string[1:], a, b)
```

```python
  else :
    return string[0] + replace(string[1:], a, b)
```

```python
replace(string, a - 1, b - 1)
```

```python
  elif string == b :
    return replace(string[len(b):], a, b)
    
  else :
    return string[0] + replace(string[1:], a, b)
```

The code below replaces all occurrences of substring `b` with substring `a` in `string`. This process is done recursively.
```python
def replace(string, a, b) :
  # Base case
  if not string:
    return ""
  
  # Recursive case
  ______________________________________________

```
What should the **recursive case** of the following code be?

The code below replaces all occurrences of substring <code>b</code> with substring <code>a</code> in <code>string</code>. This process is done recursively.
<pre><code class="language-python">def replace(string, a, b) :
 # Base case
 if not string:
 return &quot;&quot;
 
 # Recursive case
 ______________________________________________

</code></pre>
What should the recursive case of the following code be?

```python
helperFunction(string, low + 1, high - 1)
```

```python
helperFunction(string, low + 1, high)
```

```python
helperFunction(string, low, high - 1)
```

Write the **recursive case** for the following function that checks whether or not a string is a **Palindrome**.
```python
def isPalindrome(string) :
 return helperFunction(string, 0, len(string) - 1)

def helperFunction(string, low, high) :
 	# Base case1
 if high <= low : 
 return True
 
 # Base case2
 elif string[low] != string[high] : 
 return False
 
 # Recursive case
 else:
 return ____________________________
```

Write the recursive case for the following function that checks whether or not a string is a Palindrome.
<pre><code class="language-python">def isPalindrome(string) :
 return helperFunction(string, 0, len(string) - 1)

def helperFunction(string, low, high) :
 	# Base case1
 if high &lt;= low : 
 return True
 
 # Base case2
 elif string[low] != string[high] : 
 return False
 
 # Recursive case
 else:
 return ____________________________
</code></pre>

Select the **line number** of the erroneous code line

```python
1. def recursiveLength(testVariable) : 
2.  # Condition if not testVariable
3.   if not testVariable:
4.      return 1
5.
6.	# Base Case	
7.	if (testVariable == "") : 
8.		return 0
9.
10.	# Recursive Case
11.	else : 
12.		return 1 + recursiveLength(testVariable[1:]) 
```

Select the line number of the erroneous code line
<pre><code class="language-python">1. def recursiveLength(testVariable) : 
2. # Condition if not testVariable
3. if not testVariable:
4. return 1
5.
6.	# Base Case	
7.	if (testVariable == &quot;&quot;) : 
8.		return 0
9.
10.	# Recursive Case
11.	else : 
12.		return 1 + recursiveLength(testVariable[1:]) 
</code></pre>

What is the output of the following code?
```python
def foo(string) :
    if not string :
        return 0

    return (1 if string[0] in 'aeiouAEIOU' else 0) + foo(string[1:])
        
# Driver Code
print(foo("thisisrecursion"))
```

What is the output of the following code?
<pre><code class="language-python">def foo(string) :
 if not string :
 return 0

 return (1 if string[0] in 'aeiouAEIOU' else 0) + foo(string[1:])
 
# Driver Code
print(foo(&quot;thisisrecursion&quot;))
</code></pre>

A Palindrome is a string that reads the same backward and forwards.

A Palindrome is a string that reads the same backward and forwards.

This Quiz will take approximately 10 minutes.

This Quiz will take approximately 10 minutes.

---
In the next chapter, we will learn how to address arrays using recursion.

Recursion Fundamentals

Iteration Vs. Recursion

Recursion with Numbers

Recursion with Strings

Recursion with Arrays

Recursion with Data Structures

Conclusion

Test Your Knowledge 4