python

Write a function left_sublist_sum(lst, m), that returns the first sublist of lst that sums exactly to m. Otherwise, it should return None if no such sublist exists.

Function header: left_sublist_sum(lst, m)

Input: a list lst with numeric elements and a number m.

Output: the first (left-most) sublist that sums exactly to m or None if no such sublist exists.

Examples:

>>> left_sublist_sum([ 1, 8, 5, 10], 9)

[1, 8]

>>> left_sublist_sum([ -5, -2, 4, 11], 15)

[4, 11]

>>> left_sublist_sum([3, 7, 2, 3], 12)

[3, 7, 2]

>>> left_sublist_sum([5, 9, -1, 2], 9)

[9]

>>> left_sublist_sum([5, 9, -1, 2], 7)

None