Python Recursion

1) Simple Countdown

Write a recursive function countdown(n) that prints numbers from n down to 1.

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2) Sum of First N Numbers

Write a recursive function that returns the sum of 1 through n.

Example: sum_n(4) → 10

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3) Factorial

Write a recursive function factorial(n) that returns n!.

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4) Print Hello N Times

Write a recursive function print_hello(n) that prints “Hello” n times.

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5) Reverse Print

Write a recursive function that prints a string backwards.

Example: input “cat” → output t a c

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6) Sum of List Elements

Write a recursive function sum_list(lst) that returns the sum of all elements in a list.

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7) Count Digits

Write a recursive function that returns the number of digits in a positive integer.

Example: input 123 → output 3

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8) Check Palindrome

Write a recursive function that checks if a word is a palindrome.

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9) Find Maximum in List

Write a recursive function to find the largest number in a list.

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10) Fibonacci (Simple)

Write a recursive function fib(n) that returns the nth Fibonacci number.

(Note: beginner version without optimization.)

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11) Print Numbers from 1 to N

Write a recursion that prints numbers in increasing order.

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12) List Length

Write a recursive function to return the length of a list.

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13) Count Occurrences

Write a recursive function to count how many times a given value appears in a list.

Example: count_value([1,2,1,3], 1) → 2

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14) Power Function

Write a recursive function power(base, exp) to compute base**exp.

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15) GCD Using Recursion

Write a recursive function to find the greatest common divisor of two numbers.

(Using Euclid’s algorithm.)

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16) Sum of Even Numbers in List

Write a recursive function to return the sum of all even numbers in a list.

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17) Print Nested List

Given a nested list of numbers, use recursion to flatten and print all numbers.

Example: [1, [2, 3], 4] → 1 2 3 4

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18) Countdown with Steps

Write a recursive function countdown(n, step) that prints numbers from n down to 0 using a fixed step.

Example: countdown(10, 2) → 10 8 6 4 2 0

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19) Replace Character Recursively

Write a recursive function to replace all instances of a character in a string.

Example: replace every “a” with “@”.

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20) Sum of Digits

Write a recursive function that returns the sum of digits of a number.

Example: sum_digits(123) → 6

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🧠 Recursion Concepts Practiced

Pattern	What You Learn
Base Case	Needed to stop recursion
Recursive Step	Function calls itself
Return vs Print	Understanding when to return values
Working with Strings & Lists	Using slicing & indexing
Mathematical Recursion	Math problems solved by recursive logic

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📌 Example Structure

Simple Recursion Template

def func(n):
    if n == 0:
        return          # base case
    print(n)
    func(n-1)          # recursive call

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