input: Write a function to merge two sorted lists into one sorted list.
  
  def merge_sorted_lists(listA, listB): 
      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
  
  assert merge_sorted_lists([2, 4, 6], [3, 5, 7])==[2, 3, 4, 5, 6, 7]
  assert merge_sorted_lists([1, 2, 3], [4, 5, 6])==[1, 2, 3, 4, 5, 6]
  assert merge_sorted_lists([], [1, 2, 3])==[1, 2, 3]
  assert merge_sorted_lists([1, 3, 5], [2, 4, 6])==[1, 2, 3, 4, 5, 6]
  assert merge_sorted_lists([1], [])==[1]
output: def merge_sorted_lists(listA, listB): 
      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