midpoint_sum.py

'''
Midpoint Sum

For a given list of integers, return the index of the element where the sums of the integers to the left and right of the current element are equal.

Ex:

ints = [4, 1, 7, 9, 3, 9]
# Since 4 + 1 + 7 = 12 and 3 + 9 = 12, the returned index would be 3

ints = [1, 0, -1]
# Returns None/nil/undefined/etc (depending on the language) as there
# are no indices where the left and right sums are equal

Here are the 2 important rules:

The element at the index to be returned is not included in either of the sum calculations!
Both the first and last index cannot be considered as a "midpoint" (So None for [X] and [X, X])
'''

# use reduce to solve the sum
def midpoint_sum(ints):
    for i in range(1, len(ints)-1):
        if reduce(lambda x,y:x+y, ints[:i], 0) == reduce(lambda x,y:x+y, ints[i+1:], 0):
            return i
    return None

if __name__ == '__main__':
    print midpoint_sum([4,1,7,9,3,9])