input: Write a function to merge two sorted lists into one sorted list.
  assert merge_lists([1, 4, 5, 6], [2, 3, 7, 8]) == [1, 2, 3, 4, 5, 6, 7, 8]
  assert merge_lists([1, 3, 5], [2, 4, 6]) == [1, 2, 3, 4, 5, 6]
  assert merge_lists([1, 2, 3], []) == [1, 2, 3]
  assert merge_lists([], [4, 5, 6]) == [4, 5, 6]
output: def merge_lists(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