python算法：Equal Sides Of An Array

You are going to be given an array of integers. Your job is to take that array and find an index N where the sum of the integers to the left of N is equal to the sum of the integers to the right of N. If there is no index that would make this happen, return -1.

For example:
Let's say you are given the array {1,2,3,4,3,2,1}:
Your function will return the index 3, because at the 3rd position of the array, the sum of left side of the index ({1,2,3}) and the sum of the right side of the index ({3,2,1}) both equal 6.

Let's look at another one.
You are given the array {1,100,50,-51,1,1}:
Your function will return the index 1, because at the 1st position of the array, the sum of left side of the index ({1}) and the sum of the right side of the index ({50,-51,1,1}) both equal 1.

Last one:
You are given the array {20,10,-80,10,10,15,35}
At index 0 the left side is {}
The right side is {10,-80,10,10,15,35}
They both are equal to 0 when added. (Empty arrays are equal to 0 in this problem)
Index 0 is the place where the left side and right side are equal.

Note: Please remember that in most programming/scripting languages the index of an array starts at 0.

我的解法：

def find_even_index(arr):
    zrr=[]
    for i in range(len(arr)):
        if i==0:
            if sum(zrr)==sum(arr)-arr[0]:
                return i
        elif i>=1:
            zrr.append(arr[i-1])
            if sum(zrr)==sum(arr)-arr[i]-sum(zrr):
                return i
    else:
        return -1

大神解法：【爲什麼當時沒想到對列表取切片求和呢！！】

def find_even_index(arr):
    for i in range(len(arr)):
        if sum(arr[:i]) == sum(arr[i+1:]):
            return i
    return -1