前三个数的和

数组是 17、24、15、13、23。运行下面这段程序：

def lowbit(x):
    return x & (-x)

def fen_build(a):
    n = len(a)
    t = [0] * (n + 1)
    for i in range(1, n + 1):
        j = i
        while j <= n:
            t[j] += a[i - 1]
            j += lowbit(j)
    return t

def fen_prefix(t, i):
    s = 0
    while i > 0:
        s += t[i]
        i -= lowbit(i)
    return s

A = [17, 24, 15, 13, 23]
t = fen_build(A)
print(fen_prefix(t, 3))