Merge into the first list

a is a sorted list that has extra zeros at the end: exactly enough room for all of b, another sorted list. Merge b into a so that a ends up sorted, without making a new list. Filling from the front would overwrite values of a not yet placed, so fill from the back: compare the last real value of a with the last value of b, put the larger in the last slot, and move inward.

Write merge_into(a, m, b) where m is how many real values a has at the start, so len(a) is always m + len(b). Modify a in place and return it.

merge_into([1, 3, 5, 0, 0], 3, [2, 4])  ->  [1, 2, 3, 4, 5]

def merge_into(a, m, b):
    ...