和不小于 50 的最短窗口有多长

数组是 [17, 8, 15, 13, 23, 24, 19]。运行下面这段程序：

def min_len(a, target):
    lo = 0
    s = 0
    best = len(a) + 1
    for hi in range(len(a)):
        s += a[hi]
        while s >= target:
            if hi - lo + 1 < best:
                best = hi - lo + 1
            s -= a[lo]
            lo += 1
    return best

print(min_len([17, 8, 15, 13, 23, 24, 19], 50))