projecteuler---->problem=25----1000-digit Fibonacci number

问题描述：

The Fibonacci sequence is defined by the recurrence relation:

F_n = F_n1 + F_n2,
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?

翻译：

Fibonacci数列的定义如下：

F_n
= F_n-1
+ F_n-2，且F₁
= 1, F₂
= 1。

于是前12个Fibonacci数就是：

• F₁
  = 1
• F₂
  = 1
• F₃
  = 2
• F₄
  = 3
• F₅
  = 5
• F₆
  = 8
• F₇
  = 13
• F₈
  = 21
• F₉
  = 34
• F₁₀
  = 55
• F₁₁
  = 89
• F₁₂
  = 144

第十二项F₁₂，是第一个达到三位数的Fibonacci数。

第一个达到1000位的Fibonacci数又是多少呢？

解答：

def isok(a):
    count=0
    while a>0:
        a/=10
        count+=1
    if count==1000:
        return True
    return False

a,b,c=1,1,0
count=2
while True:
    c=a+b
    count+=1
    if isok(c):
        break
    a=b
    b=c
print count

projecteuler---->problem=25----1000-digit Fibonacci number