用Python解答 ProjectEuler問題(2)

E002

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, ...

Find the sum of all the even-valued terms in the sequence which do not exceed four million.

找出不大于4,000,000的斐波那契數列中偶數值的總和。

def problem2():
    a, b, c = 1, 2, 4*10**6
   
    #掐頭
    if 0==a%2:
        sum = 2*a
    elif 0==b%2:
        sum = 2*b
    else:
        sum = 0
       
    while b<=c:
        a, b = b, a+b
        sum += b
   
    #去尾
    if 0!=b%2:
        sum -= b
        if 0!=a%2:
            sum -= a
           
    return sum/2

if __name__=='__main__':
    print str(problem2())