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.

Solving Problems with Recursion 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` using recursion:
```python
def replace(string, a, b) :
  # Base case
  if not string:
    return ""
  
  # Recursive case
  ______________________________________________

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

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

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

```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 for checking whether 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 for checking whether 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 maximum 10 minutes.

This Quiz will take maximum 10 minutes.

---
In the next chapter, we will be learning how to deal with 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

Quiz on Recursion with Strings