input: Write a function to merge two sorted lists.
  assert merge_sorted_lists([2,4,6], [3,5,7]) == [2, 3, 4, 5, 6, 7]
  assert merge_sorted_lists([1,3,5], [2,4,6]) == [1, 2, 3, 4, 5, 6]
  assert merge_sorted_lists([2,4,6], []) == [2, 4, 6]
  assert merge_sorted_lists([], [3,5,7]) == [3, 5, 7]
  assert merge_sorted_lists([], []) == []
output: def merge_sorted_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
      merged_list.extend(list1[i:])
      merged_list.extend(list2[j:])
      return merged_list