create and return a merged list of all the elements in sorted order
result = []
while list1 and list2:
result.append((list1 if list1[-1] > list2[-1] else list2).pop(-1))
if len(list1):
result += list1[-1::-1]
if len(list2):
result += list2[-1::-1]
return result[-1::-1]