input: Write a function to merge two lists of integers into one sorted list without using the in-built sort function. The function should return the merged sorted list.
  assert merge_sort([3, 5, 9, 1, 8], [4, 11, 7, 2])==[1, 2, 3, 4, 5, 7, 8, 9, 11]
  assert merge_sort([1, 3, 5], [2, 4, 6])==[1, 2, 3, 4, 5, 6]
  assert merge_sort([], [1, 2, 3])==[1, 2, 3]
  assert merge_sort([10], [5])==[5, 10]
  assert merge_sort([], [])==[]
output: def merge_sort(list1, list2):
      merged_list = []
      i, j = 0, 0
      while i < len(list1) and j < len(list2):
          if list1[i] < list2[j]:
              merged_list.append(list1[i])
              i += 1
          else:
              merged_list.append(list2[j])
              j += 1
      while i < len(list1):
          merged_list.append(list1[i])
          i += 1
      while j < len(list2):
          merged_list.append(list2[j])
          j += 1
      return merged_list