Top 35 Fibonacci Sequence Interview Questions in 2025

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1. What is the Fibonacci sequence?

The Fibonacci Sequence is a series of numbers where each number $F(n)$ is the sum of the two preceding ones, often starting with 0 and 1. That is:

$$ F(n) = F(n-1) + F(n-2) $$

with initial conditions

$$ F(0) = 0, \quad F(1) = 1 $$

Golden Ratio

The ratio of consecutive Fibonacci numbers approximates the Golden Ratio ($\phi \approx 1.6180339887$):

$$ \lim_{{n \to \infty}} \frac{{F(n+1)}}{{F(n)}} = \phi $$

Real-World Occurrences

The sequence frequently manifests in nature, such as in flower petals, seedhead spirals, and seashell growth patterns.

Visual Representation

Code Example: Calculating The Nth Fibonacci Number

Here is the Python code:

# Using recursion
def fibonacci_recursive(n):
    if n <= 0: return 0
    elif n == 1: return 1
    return fibonacci_recursive(n-1) + fibonacci_recursive(n-2)
    
# Using dynamic programming for efficiency
def fibonacci_dynamic(n):
    fib = [0, 1]
    for i in range(2, n+1):
        fib.append(fib[-1] + fib[-2])
    return fib[n]

2. Write a function to calculate the nth Fibonacci number using a recursive approach.

Problem Statement

The task is to write a function that returns the $n$th Fibonacci number using a recursive approach.

Solution

The naive recursive solution for the Fibonacci series, while easy to understand, is inefficient due to its exponential time complexity of $O(2^n)$.

Dynamic Programming methods like memoization and tabulation result in optimized time complexity.

Algorithm Steps

Check for the base cases, i.e., if $n$ is 0 or 1.
If not a base case, recursively compute $F(n-1)$ and $F(n-2)$.
Return the sum of the two recursive calls.

Implementation

Here is the Python code:

def fib_recursive(n):
    if n <= 1:
        return n
    return fib_recursive(n-1) + fib_recursive(n-2)

Complexity Analysis

Time Complexity: $O(2^n)$. This is due to the two recursive calls made at each level of the recursion tree, resulting in an exponential number of function calls.
Space Complexity: $O(n)$. Despite the inefficient time complexity, the space complexity is $O(n)$ as it represents the depth of the recursion stack.

3. Provide a non-recursive implementation for generating the Fibonacci sequence to the nth number.

Problem Statement

The task is to generate the Fibonacci sequence to the $n$th number using a non-recursive approach.

Solution

While recursion offers a straightforward solution for the Fibonacci sequence, it has performance and stack overflow issues. A non-recursive approach, often based on a loop or iterative method, overcomes these limitations.

Here, I'll present both binet's formula and an iterative method as the non-recursive solutions.

Binet's Formula

$$ F(n) = \frac{{\phi^n - \psi^n}}{{\sqrt 5}} $$

where:

$\phi = \frac{{1 + \sqrt 5}}{2}$ (the golden ratio)
$\psi = \frac{{1 - \sqrt 5}}{2}$

Algorithm Steps

Compute $\phi$ and $\psi$.
Plug values into the formula for $F(n)$.

Complexity Analysis

Time Complexity: $O(1)$
Space Complexity: $O(1)$

Note: While Binet's formula offers an elegant non-recursive solution, it's sensitive to floating-point errors which can impact accuracy, especially for large $n$.

Iterative Method

Algorithm Steps

Initialize $\text{prev} = 0$ and $\text{curr} = 1$. These are the first two Fibonacci numbers.
For $i = 2$ to $n$, update $\text{prev}$ and $\text{curr}$ to be $\text{prev} + \text{curr}$ and $\text{prev}$ respectively. These become the next numbers in the sequence.

Complexity Analysis

Time Complexity: $O(n)$
Space Complexity: $O(1)$

Implementation

Here's the Python code for both methods:

Binet's Formula

import math

def fib_binet(n):
    phi = (1 + math.sqrt(5)) / 2
    psi = (1 - math.sqrt(5)) / 2
    return int((phi**n - psi**n) / math.sqrt(5))

# Output
print(fib_binet(5))  # Output: 5

Iterative Method

def fib_iterative(n):
    if n <= 1:
        return n
    prev, curr = 0, 1
    for _ in range(2, n + 1):
        prev, curr = curr, prev + curr
    return curr

# Output
print(fib_iterative(5))  # Output: 5

Both functions will return the 5th Fibonacci number.

4. What is the time complexity of the recursive Fibonacci solution, and how can this be improved?

The naive recursive implementation of the Fibonacci sequence has a time complexity of $O(2^n)$, which can be optimized using techniques such as memoization or employing an iterative approach.

Naive Recursive Approach

This is the straightforward, but inefficient, method.

Algorithm

Base Case: Return 0 if $n = 0$ and 1 if $n = 1$.
Function Call: Recur on the sum of the $n-1$ and $n-2$ elements.

Python Code

Here is the Python code:

def fibonacci_recursive(n):
    if n <= 1:
        return n
    return fibonacci_recursive(n-1) + fibonacci_recursive(n-2)

Complexity Analysis

Time Complexity: $O(2^n)$ - As each call branches into two more calls (with the exception of the base case), the number of function calls grows exponentially with $n$, resulting in this time complexity.
Space Complexity: $O(n)$ - The depth of the recursion stack can go up to $n$ due to the $n-1$ and $n-2$ calls.

Memoization for Improved Efficiency

Using memoization allows for a noticeable performance boost.

Algorithm

Initialize a cache, fib_cache, with default values of -1.
Base Case: If the $n$th value is already calculated (i.e., fib_cache[n] != -1), return that value. Otherwise, calculate the $n$th value using recursion.
Cache the Result: Once the $n$th value is determined, store it in fib_cache before returning.

Python Code

Here is the Python code:

def fibonacci_memo(n, fib_cache={0: 0, 1: 1}):
    if n not in fib_cache:
        fib_cache[n] = fibonacci_memo(n-1, fib_cache) + fibonacci_memo(n-2, fib_cache)
    return fib_cache[n]

Performance Analysis

Time Complexity: $O(n)$ - Each $n$ is computed once and then stored in the cache, so subsequent computations are $O(1)$.
Space Complexity: $O(n)$ - The space used by the cache.

Iterative Method for Superior Efficiency

The iterative approach shines in terms of time and space efficiency.

Algorithm

Initialize a and b as 0 and 1, respectively.
Loop: Update a and b to the next two Fibonacci numbers, replacing them as necessary to ensure they represent the desired numbers.
Return: a after the loop exits, since it stores the $n$th Fibonacci number.

Python Code

Here is the Python code:

def fibonacci_iterative(n):
    a, b = 0, 1
    for _ in range(n):
        a, b = b, a + b
    return a

Performance Analysis

Time Complexity: $O(n)$ - The function iterates a constant number of times, depending on $n$.
Space Complexity: $O(1)$ - The variables a and b are updated in place without utilizing any dynamic data structures or recursion stacks.

5. Describe the memoization technique as applied to the Fibonacci sequence calculation.

Memoization is a technique that makes dynamic programming faster by storing the results of expensive function calls and reusing them.

To apply memoization to the Fibonacci sequence, a list or dictionary (array or hashmap in terms of computer science) is used to store the intermediate results, essentially turning the calculation process into a more optimized dynamic programming algorithm.

Code Example: Memoized Fibonacci Calculation

Here is the Python code:

def fibonacci_memo(n, memo={0: 0, 1: 1}):
    if n not in memo:
        memo[n] = fibonacci_memo(n-1, memo) + fibonacci_memo(n-2, memo)
    return memo[n]

# Test
print(fibonacci_memo(10))  # Outputs: 55

6. Implement an iterative solution to generate the Fibonacci sequence, discussing its time and space complexity.

Problem Statement

The task is to generate the Fibonacci sequence using an iterative approach, and then analyzing its time and space complexity.

Solution

Iterative Algorithm

Initialize a = 0 and b = 1.
Use a loop to update a and b. On each iteration:
- Update a to the value of b.
- Update b to the sum of its old value and the old value of a.
Repeat the loop 'n-1' times, where 'n' is the desired sequence length.

Visual Representation

Here's how the first few Fibonacci numbers are computed:

Iteration 1: $a = 0, , b = 1, , b = 0 + 1 = 1$
Iteration 2: $a = 1, , b = 1, , b = 1 + 1 = 2$
Iteration 3: $a = 1, , b = 2, , b = 2 + 1 = 3$
Iteration 4: $a = 2, , b = 3, , b = 3 + 2 = 5$
And so on...

Complexity Analysis

Time Complexity: $O(n)$ — This is more efficient than the recursive approach which is $O(2^n)$.
Space Complexity: $O(1)$ — The space used is constant, regardless of the input 'n'.

Implementation

Here is the Python code:

def fibonacci_iterative(n):
    if n <= 0:
        return "Invalid input. n must be a positive integer."

    fib_sequence = [0] if n >= 1 else []

    a, b = 0, 1
    for _ in range(2, n+1):
        fib_sequence.append(b)
        a, b = b, a + b

    return fib_sequence

# Example usage
print(fibonacci_iterative(10)) # Output: [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

7. Explain the concept of dynamic programming as it relates to the Fibonacci sequence.

Dynamic Programming is a powerful algorithmic technique that is widely used to optimize recursive problems, like computing the Fibonacci sequence, by avoiding redundant computations.

Efficient Fibonacci Calculation

The naive method of Fibonacci computation is highly inefficient, often taking exponential time. Dynamic Programming offers better time complexity, often linear or even constant, without sacrificing accuracy.

Memoization and Caching

The most common way of optimizing Fibonacci computations is through memoization, where function calls are stored with their results for future reference. In Python, you can employ decorators or dictionaries to achieve this.

Here is the Python code:

def memoize(fib):
    cache = {}
    def wrapper(n):
        if n not in cache:
            cache[n] = fib(n)
        return cache[n]
    return wrapper

@memoize
def fib(n):
    if n < 2:
        return n
    return fib(n-1) + fib(n-2)

# Test
print(fib(10))  # 55

8. Solve the nth Fibonacci number problem using matrix exponentiation and analyze its efficiency.

Problem Statement

The task is to compute the $n$th Fibonacci number using matrix exponentiation and analyze its efficiency.

Solution

Matrix exponentiation offers an optimal $O(\log n)$ solution for the Fibonacci sequence, in contrast to the traditional recursive method that has a time complexity of $O(2^n)$.

Algorithm Steps

Represent the Fibonacci transformation as $F = \begin{bmatrix} 1 & 1 \ 1 & 0 \end{bmatrix}$.
Utilize exponentiation by squaring to efficiently compute $F^n$ for large $n$.
Extract the $n$-th Fibonacci number as the top right element of the resultant matrix.

Complexity Analysis

Time Complexity: $O(\log n)$ due to the efficiency of exponentiation by squaring.
Space Complexity: $O(\log n)$

Implementation

Here is the Python code:

import numpy as np

def matrix_power(A, n):
    if n == 1:
        return A
    if n % 2 == 0:
        half = matrix_power(A, n // 2)
        return np.dot(half, half)
    else:
        half = matrix_power(A, (n - 1) // 2)
        return np.dot(np.dot(half, half), A)

def fibonacci(n):
    if n <= 0:
        return 0
    F = np.array([[1, 1], [1, 0]])
    result = matrix_power(F, n - 1)
    return result[0, 0]

# Example usage
print(fibonacci(6))  # Output: 8

9. Can the nth Fibonacci number be found using the golden ratio, and if so, how would you implement this method?

The $n$-th term of the Fibonacci sequence can be approximated using the Golden Ratio $(\phi \approx 1.61803)$ through Binet's formula:

$$ F(n) \approx \frac{{\phi^n}}{{\sqrt{5}}} $$

Code Example: Fibonacci with Golden Ratio

Here is the Python code:

import math

def approximate_fibonacci(n):
    golden_ratio = (1 + math.sqrt(5)) / 2
    return round(golden_ratio ** n / math.sqrt(5))

# Test
print(approximate_fibonacci(10))  # Output: 55

10. Present an approach to precomputing Fibonacci numbers to answer multiple nth Fibonacci queries efficiently.

Problem Statement

The objective is to compute $\text{Fib}(n)$, the $n$th Fibonacci number, efficiently for multiple queries.

Solution

A precomputation strategy is suitable for scenarios where Fibonacci numbers are repeatedly requested over a fixed range.

Precomputation Table

Generate and store Fibonacci numbers from 0 to the highest $n$ using an array or dictionary. This operation has a time complexity of $O(n)$.
Subsequent queries are answered directly from the precomputed table with a time complexity of $O(1)$.

Complexity Analysis

Precomputation: $O(\text{max_n})$
Query time: $O(1)$

Realization

Let's consider Python as our programming language.

Code

Here is a Python function that precomputes Fibonacci numbers up to a certain limit and then returns the $n$th Fibonacci number based on the precomputed table.

def precompute_fibonacci(max_n):
    fib = [0, 1]
    a, b = 0, 1

    while b < max_n:
        a, b = b, a + b
        fib.append(b)

    return fib

def fibonacci(n, fib_table):
    return fib_table[n] if n < len(fib_table) else -1

# Usage
max_n = 100
fib_table = precompute_fibonacci(max_n)
print(fibonacci(10, fib_table))  # 55

11. How might the Fibonacci sequence be altered to start with two arbitrary initial values? Provide an algorithm for such a sequence.

Modifying the Fibonacci sequence to start with arbitrary initial values still leads to a unique sequence.

The iterative approach can handle custom starting values and compute any term in the sequence through the specified algorithm.

Algorithm: Custom Fibonacci Sequence

Input: Start values a and b (with a not equal to b) and target term n.
Check if n is 1 or 2. If it is, return the corresponding start value.
Otherwise, execute a loop n-2 times and update the start values.
Compute the n-th term once the loop concludes.

Code Example: Custom Fibonacci Sequence

Here is the Python code:

def custom_fibonacci(a, b, n):
    if n == 1:
        return a
    elif n == 2:
        return b
    
    for _ in range(n-2):
        a, b = b, a+b
    return b

# Example with start values 3 and 4 for the 6th term
result = custom_fibonacci(3, 4, 6)  # Output: 10

12. Explain an algorithm to compute the sum of the first n Fibonacci numbers without generating the entire sequence.

The Fibonacci sequence is a classic mathematical series defined by the following:

$$ F(n) = \begin{cases} 0 & \text{if } n = 0 \\ 1 & \text{if } n = 1 \\ F(n-1) + F(n-2) & \text{if } n > 1 \end{cases} $$

Direct Formulas for Sum and Generalization

Sum of First n Numbers: The sum of the first $n$ Fibonacci numbers is equal to $F(n+2) - 1$.

This can be computed using a simple, iterative function:

$$ \text{Sum}(n) = F(n+2) - 1 $$

n-th Fibonacci Number: It can be calculated using two seed values $F(0)$ and $F(1)$.

The general form is:

$$ F(n) = F(0)A + F(1)B $$

Matrix Representation: $$ \begin{bmatrix} F(n) \ F(n-1) \ \end{bmatrix}

\begin{bmatrix} 1 & 1 \ 1 & 0 \ \end{bmatrix}^{(n-1)} \begin{bmatrix} F(1) \ F(0) \ \end{bmatrix} $$

Where:

$A$ and $B$ vary based on the initial seed values.
The matrix is multiplied by itself $(n-1)$ times.

Code Example: Sum of First $n$ Fibonacci Numbers

Here is the Python code:

def fibonacci_sum(n):
    fib_curr, fib_next, fib_sum = 0, 1, 0

    for _ in range(n):
        fib_sum += fib_curr
        fib_curr, fib_next = fib_next, fib_curr + fib_next

    return fib_sum

The time complexity is $O(n)$, which is significantly better than $O(n\log n)$ when computing the $n$-th Fibonacci number using matrix exponentiation.

13. Define the Lucas sequence and detail how a program can generate it.

The Lucas Sequence, a variant of the Fibonacci sequence, starts with 2 and 1, rather than 0 and 1, and follows the same recursive structure as the classic sequence:

$$ \text{Lucas}(n) = \begin{cases} 2, & \text{if } n = 0, \\ 1, & \text{if } n = 1, \\ \text{Lucas}(n-1) + \text{Lucas}(n-2), & \text{otherwise}. \end{cases} $$

Advantages of the Lucas Sequence

Compared to the Fibonacci sequence, the Lucas sequence offers:

Simpler Recurrence Relation: The Lucas sequence uses only addition for its recursive relation, which can be computationally more efficient than Fibonacci's addition and subtraction.
Alternate Closed-Form Expression: While the closed-form formula for the $n$th term of the Fibonacci sequence involves radicals, the Lucas sequence provides an alternate expression that can be easier to work with.

14. How can the Fibonacci sequence be used to solve the tiling problem, where you need to cover a 2xn rectangle with 2x1 tiles?

The Fibonacci sequence is closely related to the problem of covering a 2xN rectangle with 2x1 tiles, often referred to as the "tiling problem". It in fact offers a direct solution to this problem.

Relationship Between Tiling and Fibonacci Sequence

Covering a 2xN rectangle $R_{1}$ with 2x1 tiles can be understood in terms of the number of ways to cover the last column, whether with a single vertical tile or two horizontal tiles:

$$ W_{1}(2xN) = W(2x(N-1)) + W(2x(N-2)) $$

This is a recursive relationship similar to the one used to define the Fibonacci numbers, making it clear that there is a connection between the two.

Code Example: Tiling a 2xN Rectangle

Here is the Python code:

def tiling_ways(n):
    a, b = 1, 1
    for _ in range(n):
        a, b = b, a + b
    return a

n = 5
ways_to_tile = tiling_ways(n)
print(f'Number of ways to tile a 2x{n} rectangle: {ways_to_tile}')

In this example, a, b = b, a + b is a compact way to update a and b. It relies on the fact that the right-hand side of the assignment is evaluated first before being assigned to the left-hand side.

This approach has a time complexity of $O(N)$ and a space complexity of $O(1)$ since it only requires two variables.

15. How to calculate large Fibonacci numbers without encountering integer overflow issues?

Fibonacci numbers grow at a rapid rate, which can lead to integer overflow. Numerous strategies exist to circumvent this issue.

Various Techniques to Handle Overflow

Using a Data Type with Larger Capacity: The long long int data type in C/C++ provides up to 19 digits of precision, thus accommodating Fibonacci numbers up to $F(92)$.
Using Built-in Arbitrary Precision Libraries: Certain programming languages such as Python and Ruby come with arbitrary-precision arithmetic support, making them suited for such computations.
Implementing Custom Arithmetic: Libraries like GMP (GNU Multiple Precision Arithmetic Library) and bigInt in Java enable the handling of arbitrary-precision operations. Additionally, rolling out a custom arithmetic procedure through arrays, linked lists, or strings is viable.

Code Example: Using `long long int` for Larger Capacity

Here is the C++ code:

  #include <iostream>
  #include <vector>
  using namespace std;
  
  long long int fibonacci(int n) {
      if (n <= 1) return n;
  
      long long int a = 0, b = 1, c;
      for (int i = 2; i <= n; ++i) {
          c = a + b;
          a = b;
          b = c;
      }
      return b;
  }

  int main() {
      int n = 100;
      cout << "Fibonacci number at position " << n << " is: " 
           << fibonacci(n) << endl;
      return 0;
  }

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