input: Write a python function to merge two sorted lists into one sorted list.
  assert merge_sorted_lists([1, 3, 5], [2, 4, 6]) == [1, 2, 3, 4, 5, 6]
  assert merge_sorted_lists([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
  assert merge_sorted_lists([1, 3, 5], [7, 8, 9]) == [1, 3, 5, 7, 8, 9]
output: def merge_sorted_lists(list1, list2):
      i, j = 0, 0
      merged_list = []
      while i < list1.length() and j < list2.length():
          if list1[i] <= list2[j]:
              merged_list.append(list1[i])
              i += 1
          else:
              merged_list.append(list2[j])
              j += 1
      while i < list1.length():
          merged_list.append(list1[i])
          i += 1
      while j < list2.length():
          merged_list.append(list2[j])
          j += 1
      return merged_list