Python Practice | Tech Bridge24

Recursion Practice

Solve problems using recursive functions with base cases and recurrence.

Factorial Using Recursion

Easy

Write a recursive function to compute the factorial of a number.

Input Format

A single line containing integer n.

Output Format

Print the factorial of n.

Sample Test Cases

Sample Input 1 5	Sample Output 1 120
Sample Input 2 6	Sample Output 2 720
Sample Input 3 3	Sample Output 3 6

Sum of First N Natural Numbers

Easy

Write a recursive function to return the sum of the first n natural numbers.

Input Format

A single line containing integer n.

Output Format

Print the sum.

Sample Test Cases

Sample Input 1 10	Sample Output 1 55
Sample Input 2 5	Sample Output 2 15
Sample Input 3 100	Sample Output 3 5050

Print Numbers 1 to N Recursively

Easy

Write a recursive function that prints numbers from 1 to n, each on a new line.

Input Format

A single line containing integer n.

Output Format

Print the numbers from 1 to n, one per line.

Sample Test Cases

Sample Input 1 5	Sample Output 1 1 2 3 4 5
Sample Input 2 3	Sample Output 2 1 2 3
Sample Input 3 1	Sample Output 3 1

Fibonacci Number

Easy

Write a recursive function to return the nth Fibonacci number (0-indexed).

Input Format

A single line containing integer n.

Output Format

Print the nth Fibonacci number.

Sample Test Cases

Sample Input 1 6	Sample Output 1 8
Sample Input 2 10	Sample Output 2 55
Sample Input 3 1	Sample Output 3 1

Sum of Digits Recursively

Easy

Write a recursive function to return the sum of digits of a number.

Input Format

A single line containing integer n.

Output Format

Print the sum of digits.

Sample Test Cases

Sample Input 1 1234	Sample Output 1 10
Sample Input 2 111	Sample Output 2 3
Sample Input 3 0	Sample Output 3 0

Power of a Number

Easy

Write a recursive function to compute x raised to the power n.

Input Format

A single line containing two space-separated integers x and n.

Output Format

Print x^n.

Sample Test Cases

Sample Input 1 2 10	Sample Output 1 1024
Sample Input 2 3 4	Sample Output 2 81
Sample Input 3 5 0	Sample Output 3 1

Reverse a String Recursively

Easy

Write a recursive function to reverse a string.

Input Format

A single line containing a string S.

Output Format

Print the reversed string.

Sample Test Cases

Sample Input 1 hello	Sample Output 1 olleh
Sample Input 2 world	Sample Output 2 dlrow
Sample Input 3 abc	Sample Output 3 cba

Count Digits Recursively

Easy

Write a recursive function to count the number of digits in a positive integer.

Input Format

A single line containing integer n.

Output Format

Print the number of digits.

Sample Test Cases

Sample Input 1 98765	Sample Output 1 5
Sample Input 2 100	Sample Output 2 3
Sample Input 3 0	Sample Output 3 1

Check Palindrome String Recursively

Easy

Write a recursive function to check whether a string is a palindrome.

Input Format

A single line containing a string S.

Output Format

Print YES if palindrome, else NO.

Sample Test Cases

Sample Input 1 racecar	Sample Output 1 YES
Sample Input 2 level	Sample Output 2 YES
Sample Input 3 python	Sample Output 3 NO

Print n Stars Recursively

Easy

Write a recursive function to print n asterisks (*) on a single line.

Input Format

A single line containing integer n.

Output Format

Print n asterisks.

Sample Test Cases

Sample Input 1 5	Sample Output 1 *****
Sample Input 2 3	Sample Output 2 ***
Sample Input 3 1	Sample Output 3 *

GCD Using Recursion

Medium

Write a recursive function to compute the GCD of two numbers using the Euclidean algorithm.

Input Format

A single line containing two space-separated integers.

Output Format

Print the GCD.

Sample Test Cases

Sample Input 1 48 18	Sample Output 1 6
Sample Input 2 54 24	Sample Output 2 6
Sample Input 3 100 10	Sample Output 3 10

Count Occurrences in a List Recursively

Medium

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

Input Format

First line: integer N. Second line: N space-separated integers. Third line: target value.

Output Format

Print the count of the target value.

Sample Test Cases

Sample Input 1 7 1 2 3 2 4 2 5 2	Sample Output 1 3
Sample Input 2 5 1 1 1 1 1 1	Sample Output 2 5
Sample Input 3 3 1 2 3 4	Sample Output 3 0

Sum of Array Recursively

Medium

Write a recursive function to return the sum of all elements in an array.

Input Format

First line: integer N. Second line: N space-separated integers.

Output Format

Print the sum.

Sample Test Cases

Sample Input 1 5 1 2 3 4 5	Sample Output 1 15
Sample Input 2 4 10 20 30 40	Sample Output 2 100
Sample Input 3 1 5	Sample Output 3 5

Decimal to Binary

Medium

Write a recursive function to convert a decimal number to its binary representation.

Input Format

A single line containing integer n.

Output Format

Print the binary representation.

Sample Test Cases

Sample Input 1 13	Sample Output 1 1101
Sample Input 2 255	Sample Output 2 11111111
Sample Input 3 10	Sample Output 3 1010

Tower of Hanoi

Medium

Write a recursive function to solve the Tower of Hanoi problem and print the moves for n disks.

Input Format

A single line containing integer n (number of disks).

Output Format

Print each move as 'Move disk d from A to C' style lines.

Sample Test Cases

Sample Input 1

Sample Output 1

Move disk 1 from A to B
Move disk 2 from A to C
Move disk 1 from B to C

Sample Input 2

Sample Output 2

Move disk 1 from A to C

Sample Input 3

Sample Output 3

Move disk 1 from A to C
Move disk 2 from A to B
Move disk 1 from C to B
Move disk 3 from A to C
Move disk 1 from B to A
Move disk 2 from B to C
Move disk 1 from A to C

Check if Array is Sorted Recursively

Medium

Write a recursive function to check whether an array is sorted in non-decreasing order.

Input Format

First line: integer N. Second line: N space-separated integers.

Output Format

Print YES if sorted, else NO.

Sample Test Cases

Sample Input 1 5 1 3 5 7 9	Sample Output 1 YES
Sample Input 2 3 3 2 1	Sample Output 2 NO
Sample Input 3 4 1 2 2 3	Sample Output 3 YES

Nth Term of a Recurrence

Medium

Write a recursive function to compute the nth term where term(n) = term(n-1) + term(n-2), with term(0)=0, term(1)=1.

Input Format

A single line containing integer n.

Output Format

Print the nth term.

Sample Test Cases

Sample Input 1 9	Sample Output 1 34
Sample Input 2 10	Sample Output 2 55
Sample Input 3 7	Sample Output 3 13

Find Maximum in Array Recursively

Medium

Write a recursive function to find the maximum element in an array.

Input Format

First line: integer N. Second line: N space-separated integers.

Output Format

Print the maximum element.

Sample Test Cases

Sample Input 1 6 4 9 2 7 5 1	Sample Output 1 9
Sample Input 2 5 8 3 9 1 7	Sample Output 2 9
Sample Input 3 2 -1 -2	Sample Output 3 -1

String Length Recursively

Medium

Write a recursive function to find the length of a string without using len().

Input Format

A single line containing a string S.

Output Format

Print the length of S.

Sample Test Cases

Sample Input 1 programming	Sample Output 1 11
Sample Input 2 abc	Sample Output 2 3
Sample Input 3	Sample Output 3 0

Generate Fibonacci Series Recursively

Medium

Write a recursive function that prints the first n Fibonacci numbers.

Input Format

A single line containing integer n.

Output Format

Print the first n Fibonacci numbers space-separated.

Sample Test Cases

Sample Input 1 7	Sample Output 1 0 1 1 2 3 5 8
Sample Input 2 5	Sample Output 2 0 1 1 2 3
Sample Input 3 1	Sample Output 3 0

N-Queens Problem

Hard

Write a recursive backtracking program to count the number of ways to place n queens on an n x n board so that no two attack each other.

Input Format

A single line containing integer n.

Output Format

Print the number of valid arrangements.

Sample Test Cases

Sample Input 1 4	Sample Output 1 2
Sample Input 2 5	Sample Output 2 10
Sample Input 3 1	Sample Output 3 1

Permutations of a String

Hard

Write a recursive function to generate all permutations of the given string.

Input Format

A single line containing a string S.

Output Format

Print each permutation on a new line, in any order.

Sample Test Cases

Sample Input 1 abc	Sample Output 1 abc acb bac bca cab cba
Sample Input 2 ab	Sample Output 2 ab ba
Sample Input 3 a	Sample Output 3 a

Subset Sum Count

Hard

Write a recursive function to count the number of subsets of an array whose sum equals a target.

Input Format

First line: integer N. Second line: N space-separated integers. Third line: target sum.

Output Format

Print the count of subsets.

Sample Test Cases

Sample Input 1 5 1 2 3 4 5 7	Sample Output 1 3
Sample Input 2 4 1 2 3 4 5	Sample Output 2 2
Sample Input 3 3 1 2 3 3	Sample Output 3 2

Combinations of K Elements

Hard

Write a recursive function to print all combinations of k elements from an array of n distinct integers.

Input Format

First line: N and k space-separated. Second line: N space-separated integers.

Output Format

Print each combination on a new line, elements space-separated.

Sample Test Cases

Sample Input 1 4 2 1 2 3 4	Sample Output 1 1 2 1 3 1 4 2 3 2 4 3 4
Sample Input 2 3 2 1 2 3	Sample Output 2 1 2 1 3 2 3
Sample Input 3 2 1 9 8	Sample Output 3 9 8

Knight's Tour Count

Hard

Write a recursive program to count the number of closed tours a knight can make on a small n x n board starting from (0,0).

Input Format

A single line containing integer n (2 ≤ n ≤ 5).

Output Format

Print the number of closed knight tours.

Sample Test Cases

Sample Input 1 5	Sample Output 1 1728
Sample Input 2 4	Sample Output 2 0
Sample Input 3 3	Sample Output 3 0

Palindrome Partitioning

Hard

Write a recursive function to print all ways to partition a string such that every part is a palindrome.

Input Format

A single line containing a string S.

Output Format

Print each partition on a new line, parts space-separated.

Sample Test Cases

Sample Input 1 aab	Sample Output 1 a a b aa b
Sample Input 2 aab	Sample Output 2 a a b aa b
Sample Input 3 aba	Sample Output 3 a b a aba

Generate All Balanced Parentheses

Hard

Write a recursive function to generate all combinations of n pairs of balanced parentheses.

Input Format

A single line containing integer n.

Output Format

Print each valid combination on a new line.

Sample Test Cases

Sample Input 1 3	Sample Output 1 ((())) (()()) (())() ()(()) ()()()
Sample Input 2 2	Sample Output 2 (()) ()()
Sample Input 3 1	Sample Output 3 ()

Binary Search Recursive

Hard

Write a recursive binary search function that returns the index of a target in a sorted array, or -1.

Input Format

First line: integer N. Second line: N space-separated sorted integers. Third line: target.

Output Format

Print the index (0-based), or -1.

Sample Test Cases

Sample Input 1 6 1 3 5 7 9 11 7	Sample Output 1 3
Sample Input 2 5 1 2 3 4 5 5	Sample Output 2 4
Sample Input 3 5 1 2 3 4 5 0	Sample Output 3 -1

Maze Paths Count

Hard

Write a recursive function to count the number of ways to reach the bottom-right cell of an m x n grid moving only right or down.

Input Format

A single line containing two space-separated integers m and n.

Output Format

Print the number of paths.

Sample Test Cases

Sample Input 1 3 3	Sample Output 1 6
Sample Input 2 2 3	Sample Output 2 3
Sample Input 3 1 1	Sample Output 3 1

Tower of Hanoi with Count

Hard

Write a recursive program to solve the Tower of Hanoi for n disks and also print the total number of moves.

Input Format

A single line containing integer n.

Output Format

Print the moves first (one per line), then 'Total moves: [count]'.

Sample Test Cases

Sample Input 1

Sample Output 1

Move disk 1 from A to C
Move disk 2 from A to B
Move disk 1 from C to B
Move disk 3 from A to C
Move disk 1 from B to A
Move disk 2 from B to C
Move disk 1 from A to C
Total moves: 7

Sample Input 2

Sample Output 2

Move disk 1 from A to B
Move disk 2 from A to C
Move disk 1 from B to C
Total moves: 3

Sample Input 3

Sample Output 3

Move disk 1 from A to C
Total moves: 1

Competitive MCQs — Recursion

Code snippets, output prediction, concepts & error spotting. Pick an answer to see instant feedback.

Score 0/ 34

What is a recursive function?

Correct!

Wrong — correct answer is .

Recursion is when a function calls itself to solve smaller subproblems.

What is the output of this code?

python

def fact(n):
    if n <= 1:
        return 1
    return n * fact(n - 1)
print(fact(5))

Correct!

Wrong — correct answer is .

fact(5) = 5 × 4 × 3 × 2 × 1 = 120.

What is the output of this code?
def fact(n):
if n <= 1:
return 1
return n * fact(n - 1)
print(fact(5))

Correct!

Wrong — correct answer is .

fact(5) = 5 × 4 × 3 × 2 × 1 = 120.

What is the base case in a recursive function?

Correct!

Wrong — correct answer is .

A base case prevents infinite recursion by providing a stopping condition.

What happens if a recursive function has no base case?

Correct!

Wrong — correct answer is .

Unbounded recursion consumes the call stack and raises RecursionError.

What is the output of this code?

python

def count(n):
    if n == 0:
        return 0
    return 1 + count(n - 1)
print(count(4))

Correct!

Wrong — correct answer is .

Each call adds 1 until n hits 0: 1+1+1+1 = 4.

What is the output of this code?
def count(n):
if n == 0:
return 0
return 1 + count(n - 1)
print(count(4))

Correct!

Wrong — correct answer is .

Each call adds 1 until n hits 0: 1+1+1+1 = 4.

Which series is classically defined recursively as F(n) = F(n-1) + F(n-2)?

Correct!

Wrong — correct answer is .

The Fibonacci sequence is the classic recursive example.

What is the output of this code?

python

def greet(n):
    if n <= 0:
        return
    print("Hi")
    greet(n - 1)
greet(3)

Correct!

Wrong — correct answer is .

greet(3) prints Hi and calls greet(2), which prints and calls greet(1)... 3 lines.

Q10

What is the output of this code?
def greet(n):
if n <= 0:
return
print("Hi")
greet(n - 1)
greet(3)

Correct!

Wrong — correct answer is .

greet(3) prints Hi and calls greet(2), which prints and calls greet(1)... 3 lines.

Q11

What is the maximum recursion depth problem in Python called?

Correct!

Wrong — correct answer is .

Python raises RecursionError when the recursion limit is exceeded.

Q12

What is the output of this code?

python

def power(b, e):
    if e == 0:
        return 1
    return b * power(b, e - 1)
print(power(2, 3))

Correct!

Wrong — correct answer is .

2 × 2 × 2 = 8; power(2,3) multiplies 2 three times.

Q13

What is the output of this code?
def power(b, e):
if e == 0:
return 1
return b * power(b, e - 1)
print(power(2, 3))

Correct!

Wrong — correct answer is .

2 × 2 × 2 = 8; power(2,3) multiplies 2 three times.

Q14

Every recursive solution can also be written using which construct?

Correct!

Wrong — correct answer is .

Any recursive function can be rewritten iteratively with loops and a stack.

Q15

What is the output of this code?
def f(n):
if n == 0:
return 0
return n + f(n - 1)
print(f(3))

Correct!

Wrong — correct answer is .

f(3) = 3 + 2 + 1 + 0 = 6.

Q16

What is the output of this code?
def rev(s):
if s == "":
return ""
return rev(s[1:]) + s[0]
print(rev("abc"))

Correct!

Wrong — correct answer is .

The recursion moves the first character to the end each step, reversing the string.

Q17

How many total calls are made by f(3) in this code?
def f(n):
if n <= 1:
return 1
return f(n - 1) + f(n - 2)
print(f(3))

Correct!

Wrong — correct answer is .

f(3) calls f(2) and f(1); f(2) calls f(1) and f(0) → 5 calls including the initial one.

Q18

What happens when recursion never reaches its base case?

Correct!

Wrong — correct answer is .

Python raises RecursionError once the recursion limit is crossed.

Q19

Which is the best definition of a base case?

Correct!

Wrong — correct answer is .

Base cases stop the chain by returning without another recursive call.

Q20

What is the output of this code?
def ack(m, n):
return m + n
print(ack(2, ack(1, 1)))

Correct!

Wrong — correct answer is .

Inner ack(1, 1) = 2, so outer ack(2, 2) = 4.

Q21

Which classic problem is naturally recursive?

Correct!

Wrong — correct answer is .

Tower of Hanoi maps directly onto recursive moves.

Q22

What is the output of this code?
def countdown(n):
if n == 0:
return
print(n, end=" ")
countdown(n - 1)
countdown(3)

Correct!

Wrong — correct answer is .

The function prints 3, recurses with 2, prints 2, recurses with 1, and stops at 0.

Q23

What is the output of this code?
def power(b, e):
if e == 0:
return 1
return b * power(b, e - 1)
print(power(3, 2))

Correct!

Wrong — correct answer is .

power(3, 2) = 3 × 3 × 1 = 9.

Q24

What is the primary risk of recursion?

Correct!

Wrong — correct answer is .

Each call consumes stack memory; very deep chains overflow it.

Q25

What is the output of this code?
def f(n):
if n == 1:
return 1
return 1 + f(n - 1)
print(f(4))

Correct!

Wrong — correct answer is .

Four recursive steps each add 1 until n reaches 1, giving 4.

Q26

Which data structure does recursion implicitly use?

Correct!

Wrong — correct answer is .

Function calls are pushed onto the call stack and popped on return.

Q27

What is the output of this code?
def gcd(a, b):
if b == 0:
return a
return gcd(b, a % b)
print(gcd(12, 8))

Correct!

Wrong — correct answer is .

Euclid's algorithm: gcd(12, 8) → gcd(8, 4) → gcd(4, 0) = 4.

Q28

What is recursion depth?

Correct!

Wrong — correct answer is .

Depth counts how many calls are currently stacked before returning.

Q29

Which change usually fixes a RecursionError?

Correct!

Wrong — correct answer is .

A correct base case or a larger sys.setrecursionlimit resolves deep recursion.

Q30

What is the output of this code?
def f(n):
if n < 2:
return n
return f(n - 1) + f(n - 2)
print(f(6))

Correct!

Wrong — correct answer is .

This is Fibonacci: 0, 1, 1, 2, 3, 5, 8 — f(6) is 8.

Q31

Which is TRUE about tail recursion in Python?

Correct!

Wrong — correct answer is .

CPython does not implement tail-call optimization, so deep tail recursion still overflows.

Q32

What is the output of this code?
def f(n):
return 1 if n == 0 else n * f(n - 1)
print(f(0))

Correct!

Wrong — correct answer is .

f(0) hits the base case immediately and returns 1.

Q33

Which type of problems suits recursion best?

Correct!

Wrong — correct answer is .

Problems like tree traversal and divide-and-conquer mirror recursion naturally.

Q34

What is the output of this code?
def f(n):
if n == 0:
return ""
return "*" + f(n - 1)
print(f(3))

Correct!

Wrong — correct answer is .

Each call prepends one star; three calls produce "***".

Python Practice Questions

Recursion Practice

Competitive MCQs — Recursion