Python Recursion

Recursion is a process where a function calls itself. It's used to solve problems that can be broken down into smaller sub-problems of the same type.

Recursion वह प्रक्रिया है जिसमें एक function खुद को call करता है। इसका उपयोग उन्हीं problems को solve करने के लिए किया जाता है जिन्हें छोटे-छोटे हिस्सों में divide किया जा सकता है।

How Recursion Works

Every recursive function must have a base condition that stops the recursion. Otherwise, it will run infinitely and cause an error.

हर recursive function में एक base condition होना चाहिए जो recursion को रोकता है, वरना infinite बार call होता रहेगा।

Advantages of Recursion

• Code becomes clean and short
• Good for problems like tree traversal, factorial, Fibonacci

Disadvantages of Recursion

• Uses more memory due to function calls
• Can cause stack overflow if base condition is not met

1. Factorial of a number

   Copydef factorial(n):
       if n == 1:
           return 1
       return n * factorial(n - 1)
   
   print("Factorial:", factorial(5))

   Output:

   Factorial: 120

2. Fibonacci series

   Copydef fib(n):
       if n <= 1:
           return n
       return fib(n-1) + fib(n-2)
   
   for i in range(6):
       print(fib(i), end=' ')

   Output:

   0 1 1 2 3 5

3. Sum of first N natural numbers

   Copydef sum_n(n):
       if n == 0:
           return 0
       return n + sum_n(n - 1)
   
   print("Sum:", sum_n(10))

   Output:

   Sum: 55

4. Reverse a string using recursion

   Copydef reverse(s):
       if len(s) == 0:
           return s
       return reverse(s[1:]) + s[0]
   
   print(reverse("hello"))

   Output:

   olleh

5. Countdown using recursion

   Copydef countdown(n):
       if n == 0:
           print("Blast off!")
       else:
           print(n)
           countdown(n - 1)
   
   countdown(5)

   Output:

   5
   4
   3
   2
   1
   Blast off!