5 Ways to Find the Minimum Number of Fibonacci Numbers to Sum Up to N in Python

💡 Problem Formulation: Given an integer n, the challenge is to find the smallest number of Fibonacci numbers whose sum is equal to n. For instance, if n = 10, the Fibonacci numbers could be 8 and 2, making the output 2 since it takes two Fibonacci numbers to add up to 10.

Method 1: Greedy Algorithm Approach

The Greedy Algorithm Approach for finding the minimum number of Fibonacci numbers to sum up to n works by starting from the largest possible Fibonacci number not greater than n and iteratively subtracts this from n and updates n to this new value, counting the numbers needed as it goes. This method is efficient and straightforward because every number can be represented as a sum of non-consecutive Fibonacci numbers (Zeckendorf’s theorem).

Here’s an example:

def generate_fib(n):
    fib_nums = [1, 1]
    while fib_nums[-1]  0:
        fib_num = fibs.pop()
        if fib_num <= n:
            n -= fib_num
            count += 1
    return count

print(min_fibonacci_nums(10))

Output: 2

This code snippet defines two functions: generate_fib(n) generates a list of Fibonacci numbers up to n, and min_fibonacci_nums(n) uses this list to find the minimum number of Fibonacci numbers that sum up to n using the Greedy Algorithm. It keeps track of the count of Fibonacci numbers used, and the process repeats until n is reduced to zero.

Method 2: Recursive Approach

The Recursive Approach involves breaking down the problem into smaller instances of itself. It finds the largest Fibonacci number smaller than or equal to n, subtracts it from n, and proceeds with recursion on the remainder n - fib. While elegant, this method is not efficient for large inputs due to the overhead of repeated calculations.

Here’s an example:

def find_closest_fib(n):
    a, b = 1, 1
    while b <= n:
        a, b = b, a + b
    return a

def min_fibonacci_nums_recursive(n):
    if n == 0:
        return 0
    fib = find_closest_fib(n)
    return 1 + min_fibonacci_nums_recursive(n - fib)

print(min_fibonacci_nums_recursive(10))

Output: 2

This recursive function min_fibonacci_nums_recursive(n) calls itself with the reduced problem size n - find_closest_fib(n) repeatedly until n reaches zero. It relies on the function find_closest_fib(n) to get the largest Fibonacci number not exceeding n and increments its count each time it’s called.

Method 3: Dynamic Programming

In Dynamic Programming, the algorithm remembers past results (caching) to avoid repeated calculations, which greatly improves efficiency for larger values of n. This approach builds solutions for all numbers up to n by adding 1 to the minimum number found for previous numbers.

Here’s an example:

def min_fibonacci_nums_dp(n):
    fibs = generate_fib(n)
    min_nums = [0] * (n + 1)
    for f in fibs:
        if f  0 and min_nums[i - f] > 0:
                if min_nums[i] == 0 or min_nums[i] > min_nums[i - f] + 1:
                    min_nums[i] = min_nums[i - f] + 1
    return min_nums[n]

print(min_fibonacci_nums_dp(10))

Output: 2

This code uses a list min_nums to store the minimum number of Fibonacci numbers needed for every number up to n. It iteratively computes the minimum count for each number by checking previously computed results, leading to a more optimal solution without redundant calculations.

Method 4: Matrix Exponentiation

Matrix Exponentiation is a highly efficient method for computing Fibonacci numbers. It uses the property of Fibonacci numbers where a matrix, when raised to a power, yields another matrix with the next Fibonacci numbers. This method is not directly related to finding the minimum count but can be used to efficiently generate Fibonacci numbers when paired with another method.

Here’s an example:

import numpy as np

def matrix_power(matrix, n):
    result = np.identity(len(matrix), dtype=int)
    while n > 0:

        if n % 2 == 1:
            result = np.matmul(result, matrix)
        n //= 2
        matrix = np.matmul(matrix, matrix)
    return result

def fib_matrix(n):
    matrix = np.array([[1, 1], [1, 0]], dtype=int)
    return matrix_power(matrix, n)[0, 1]

# Use fib_matrix in conjunction with a method like the Greedy Algorithm

This snippet introduces a matrix exponentiation function matrix_power(matrix, n) to compute powers of a matrix. The fib_matrix(n) function uses this to calculate the nth Fibonacci number. It should be integrated with another algorithm, such as the Greedy Algorithm, to determine the minimum number of Fibonacci numbers that sum up to n.

Bonus One-Liner Method 5: List Comprehension and Greedy Approach

This Bonus One-Liner is a compact version of the Greedy Algorithm that leverages Python’s list comprehension and the greedy property. It does not provide the array of Fibonacci numbers but directly computes the count in one single line of code.

Here’s an example:

from bisect import bisect_right

n = 10
fib_nums = [1, 1]
while fib_nums[-1] = fib_nums[bisect_right(fib_nums, n -= fib) - 1] and (n := n - fib_nums[bisect_right(fib_nums, n -= fib) - 1]) for fib in fib_nums))

Output: 2

This one-liner finds the minimum number of Fibonacci numbers to sum to n using a complex list comprehension and the bisect_right function from the bisect module. It’s not easy to understand or maintain, but it is concise.

Summary/Discussion

• Method 1: Greedy Algorithm Approach. Good performance. Simple and elegant.
• Method 2: Recursive Approach. Simple conceptual understanding. Not efficient for large n due to high time complexity.
• Method 3: Dynamic Programming. More efficient for larger n. Requires additional space for caching.
• Method 4: Matrix Exponentiation. Highly efficient for calculating Fibonacci numbers. Only used for number generation, not directly for the sum problem.
• Bonus Method 5: One-Liner. Compact and uses a clever combination of list comprehension and greedy strategy. Difficult to read and understand.