Euler-Problems/problem25.py at master · respectus/Euler-Problems · GitHub

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"""
1000-digit Fibonnaci Number
Project Euler Problem #25
by Muaz Siddiqui

The Fibonacci sequence is defined by the recurrence relation:

Fn = Fn−1 + Fn−2, where F1 = 1 and F2 = 1.
Hence the first 12 terms will be:

F1 = 1
F2 = 1
F3 = 2
F4 = 3
F5 = 5
F6 = 8
F7 = 13
F8 = 21
F9 = 34
F10 = 55
F11 = 89
F12 = 144
The 12th term, F12, is the first term to contain three digits.

What is the first term in the Fibonacci sequence to contain 1000 digits?
"""
def memoize(f):
    cache = {}
    return lambda *args: cache[args] if args in cache else cache.update({args: f(*args)}) or cache[args]

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

def answer():
    n = 500
    while len(str(fib(n))) < 1000:
        n+=1
    return n