Merge pull request #671 from codewithdhruba01/doc/python-recursion

docs(python): add detailed chapter on Recursion in Python

Author: sanjay-kv

docs/python/python-recursion.md

@@ -0,0 +1,172 @@
+---
+id: python-recursion
+title: Recursion in Python
+sidebar_label: Recursion in Python
+sidebar_position: 12
+tags:
+  [
+    Python,
+    List in Python,
+    Introduction of python,
+    Python Syntax,
+    Variables,
+    Operators,
+    Type Casting,
+    String,
+    Tuple in Python
+  ]
+  
+---
+
+# Recursion in Python
+
+**Recursion** is a programming technique where a function calls itself directly or indirectly to solve a problem.  
+It is often used to break down complex problems into smaller, simpler sub-problems.
+
+---
+
+## Basic Structure of Recursion
+
+Every recursive function has two main parts:
+
+1. **Base Case** → Stops the recursion (prevents infinite calls).  
+2. **Recursive Case** → Function calls itself with modified input.
+
+**Example:**
+```python
+def countdown(n):
+    if n == 0:   # Base case
+        print("Time's up!")
+    else:        # Recursive case
+        print(n)
+        countdown(n-1)
+
+countdown(5)
+```
+
+```python
+Output:
+# 5
+# 4
+# 3
+# 2
+# 1
+# Time's up!
+````
+
+## Factorial using Recursion
+
+Factorial of `n` → `n! = n × (n-1) × (n-2) × ... × 1`
+
+```python
+def factorial(n):
+    if n == 0 or n == 1:  # Base case
+        return 1
+    else:                 # Recursive case
+        return n * factorial(n-1)
+
+print(factorial(5))  # Output: 120
+```
+
+## Fibonacci Series using Recursion
+
+Fibonacci sequence → 0, 1, 1, 2, 3, 5, 8, ...
+
+```python
+def fibonacci(n):
+    if n <= 1:   # Base case
+        return n
+    else:        # Recursive case
+        return fibonacci(n-1) + fibonacci(n-2)
+
+for i in range(7):
+    print(fibonacci(i), end=" ")
+# Output: 0 1 1 2 3 5 8
+```
+
+
+## Recursion vs Iteration
+
+* **Iteration (loops):** Uses `for` or `while` loops.
+* **Recursion:** Function calls itself.
+
+**Example: Sum of first n numbers**
+
+Recursive:
+
+```python
+def sum_recursive(n):
+    if n == 0:
+        return 0
+    return n + sum_recursive(n-1)
+
+print(sum_recursive(5))  # Output: 15
+```
+
+Iterative:
+
+```python
+def sum_iterative(n):
+    total = 0
+    for i in range(1, n+1):
+        total += i
+    return total
+
+print(sum_iterative(5))  # Output: 15
+```
+
+
+## Advantages of Recursion
+
+ Makes code **shorter and cleaner**
+ Useful for problems naturally defined recursively (factorial, Fibonacci, tree traversal, divide and conquer algorithms)
+
+
+## Disadvantages of Recursion
+
+ **Slower execution** than iteration (due to repeated function calls)
+ **Memory usage is high** (function calls are stored in the call stack)
+ Risk of **stack overflow error** if base case is missing
+
+
+## Tail Recursion in Python
+
+Tail recursion is when the **recursive call is the last statement** in the function.
+Unlike some languages, Python **does not optimize tail recursion**, so deep recursion may cause errors.
+
+```python
+def tail_sum(n, accumulator=0):
+    if n == 0:
+        return accumulator
+    return tail_sum(n-1, accumulator+n)
+
+print(tail_sum(5))  # Output: 15
+```
+
+
+## Practical Example: Binary Search (Recursive)
+
+```python
+def binary_search(arr, target, low, high):
+    if low > high:
+        return -1   # Not found
+
+    mid = (low + high) // 2
+
+    if arr[mid] == target:
+        return mid
+    elif arr[mid] < target:
+        return binary_search(arr, target, mid+1, high)
+    else:
+        return binary_search(arr, target, low, mid-1)
+
+nums = [1, 3, 5, 7, 9, 11]
+print(binary_search(nums, 7, 0, len(nums)-1))  # Output: 3
+```
+
+## Conclusion
+
+* Recursion is a function calling itself to solve smaller sub-problems.
+* Every recursive function must have a **base case** to avoid infinite calls.
+* Useful for problems like factorial, Fibonacci, searching, sorting, and tree/graph traversal.
+* While recursion makes code elegant, it may be slower and consume more memory than iteration.

docs/python/setup-environment.md

@@ -2,7 +2,7 @@
 id: setup-environment
 title: Setting up your development environment
 sidebar_label: Setting up environment
-sidebar_position: 12
+sidebar_position: 13
 tags:
   [
     html,