input: Write a python function to merge two sorted lists into one sorted list.
  assert merge_sorted_lists([1, 5, 11, 15], [3, 10, 12])==[1, 3, 5, 10, 11, 12, 15]
  assert merge_sorted_lists([1, 2, 4], [3, 5, 6])==[1, 2, 3, 4, 5, 6]
  assert merge_sorted_lists([], [1, 2, 3])==[1, 2, 3]
  assert merge_sorted_lists([1, 2, 3], [])==[1, 2, 3]
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