def hb(list1,list2): result = [] while list1 and list2: if list1[0] < list2[0]: result.append(list1[0]) del list1[0] else: result.append(list2[0]) del list2[0] if list1: result.extend(list1) if list2: result.extend(list2) print(result) return result list1 = [3,4,7,9,11] list2 = [1,2,5,8,13,20] hb(list1, list2) >>> [1, 2, 3, 4, 5, 7, 8, 9, 11, 13, 20]