求中心下标

def fun1(li: list):
    if len(li) == 1: return 1
    sum_num = sum(li)
    res = 0
    for i in range(len(li)):
        res += li[i]
        if res == sum_num:
            return i
        sum_num -= li[i]

    return -1


def fun2(li: list):
    sum_num = sum(li)
    res = 0
    for i in range(len(li)):
        if 2 * res + li[i] == sum_num:
            return i
        res += li[i]
    return -1


li = [1, 7, 3, 6, 5, 6]
# li = [1,1]
print(fun2(li))