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原题地址 http://projecteuler.net/problem=2

Even Fibonacci numbers

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, …

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

偶数斐波纳契数
斐波那契数列中的每一个数等于前面两个数的和，从1和2开始，前面十项为：

1, 2, 3, 5, 8, 13, 21, 34, 55, 89，。。。

考虑斐波那契数中不超过4000000的数，求这些数中所有偶数的和。

解法：

这题不多说，关键是生成斐波那契数列，这里使用了一个迭代器的技巧。代码如下：

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#!/usr/bin/python
# -*- coding:utf-8 -*-
'''
Created on 2013-4-26

@author: shilong
@email: long470884130@163.com
'''

def fibonacci():
    """一个迭代函数，每次得到一个斐波那契数,依次将得到1, 2, 3, 5..."""
    a = 1
    b = 1
    while True:
        yield a
        a, b = a + b, a

if __name__ == "__main__":
    fi = fibonacci()
    s = 0
    while True:
        n = next(fi)
        if n >= 4000000:
            break
        if n % 2 == 0:
            s += n
    print s

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