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]
      """
      merged_list = []
      ptrA = 0
      ptrB = 0
      while ptrA < len(listA) and ptrB < len(listB): 
          if listA[ptrA] <= listB[ptrB]: 
              merged_list.append(listA[ptrA]) 
              ptrA += 1
          else: 
              merged_list.append(listB[ptrB]) 
              ptrB += 1
      while ptrA < len(listA): 
          merged_list.append(listA[ptrA]) 
          ptrA += 1
      while ptrB < len(listB):
          merged_list.append(listB[ptrB])
          ptrB += 1
      return merged_list
output:     
      merged_list = []
      ptrA = 0
      ptrB = 0
      while ptrA < len(listA) and ptrB < len(listB): 
          if listA[ptrA] <= listB[ptrB]: 
              merged_list.append(listA[ptrA]) 
              ptrA += 1
          else: 
              merged_list.append(listB[ptrB]) 
              ptrB += 1
      while ptrA < len(listA): 
          merged_list.append(listA[ptrA]) 
          ptrA += 1
      while ptrB < len(listB):
          merged_list.append(listB[ptrB])
          ptrB += 1
      return merged_list
  