This one, actually called “Equal sides of an array” threw me for a loop because I hadn’t realized we were excluding the i-th element for each possible index where we would try splitting the array. That induces some off-by-one-ish errors, which is the main interesting part.

This is a nice first approach, and it gets the goal:

def find_even_index(arr):
    if len(arr) == 0: return True
    for i in range(len(arr)):
        if sum(arr[:i]) == sum(arr[i+1:]):
            return i
    return -1

The slices-and-sums can be helper functions.

The natural sequel is to not repeat the addition so many times (so, what’s the runtime complexity of the first solution?). That requires some careful thinking and avoiding off-by-one errors.

def find_even_index(arr):
    if len(arr) == 0: return True
    leftSum = 0
    rightSum = sum(arr[1:])
    if leftSum == rightSum:
        return 0
    for i in range(1, len(arr)):
        leftSum += arr[i-1]
        rightSum -= arr[i]
        if leftSum == rightSum:
            return i
    return -1

Take-Aways

This problem seems to hit a lot of right notes: It’s approachable, it has trickiness that requires careful debugging thought, it has plausible applicability (partitioning resources evenly), and keeps the door open to optimizations. Nice! For this and other array questions, I collect them here.