Project Euler 25

• 問題

Problem 25：1000-digit Fibonacci number
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 index of the first term in the Fibonacci sequence to contain 1000 digits?

• 解答例

f1 = 1
f2 = 1
i = 3
while len(str(f1 + f2)) < 1000:
    f1, f2 = f2, (f1 + f2)
    i += 1
    
print(i)