input: def merge_sorted_lists(listA, listB): 
      """
      Merge two sorted lists into a single sorted list.
      >>> merge_sorted_lists([2, 4, 6], [3, 5, 7])
      [2, 3, 4, 5, 6, 7]
      """
output:     merged_list = []
      i, j = 0, 0
      while i < len(listA) and j < len(listB):
          if listA[i] < listB[j]:
              merged_list.append(listA[i])
              i += 1
          else:
              merged_list.append(listB[j])
              j += 1
      merged_list.extend(listA[i:])
      merged_list.extend(listB[j:])
      return merged_list