def merge_list(list_1, list_2):
     merged_list = list_1 + list_2
     merged_list.sort()
     return(merged_list)

list_1 = [20, 18, 9, 51, 48, 31]
list_2 = [28, 33, 3, 22, 15, 20]
print("第一个列表是：")
print(list_1)
print("第二个列表是：")
print(list_2)
print(merge_list(list_1, list_2))

第一个列表是：
[20, 18, 9, 51, 48, 31]
第二个列表是：
[28, 33, 3, 22, 15, 20]
[3, 9, 15, 18, 20, 20, 22, 28, 31, 33, 48, 51]