求解列表特定位置的索引，使左右两边之和相等

def find_index(li):
    if not li or len(li) == 0:
        return
    li_sum = sum(li)
    left_total = 0
    for i in range(len(li)):
        left_total += li[i]
        right_total = li_sum - left_total
        if left_total == right_total:
            return i