三个数组，各能不能分成两半

"划分问题"：能不能把一个数组分成两堆，两堆的和相等？三个数组 [17, 24, 15, 13, 23]、[10, 20, 15, 5]、[17, 24, 15, 13, 23, 3] 各判一次。注意 can_split 一行新算法都没写，只是把输入交给了 subset_sum：

NO = [17, 24, 15, 13, 23]
YES = [10, 20, 15, 5]
ODD = [17, 24, 15, 13, 23, 3]

def subset_sum(arr, target):
    """老朋友：能不能挑出一个子集，和正好等于 target"""
    reach = {0}
    for x in arr:
        reach |= {r + x for r in reach}
    return target in reach

def can_split(arr):
    s = sum(arr)
    if s % 2 == 1:
        return False
    return subset_sum(arr, s // 2)

print("/".join("能" if can_split(x) else "不能" for x in (NO, YES, ODD)))