nums = [5, 12, 7, 9, 21, 7, 14]
count_seven = nums.count(7)
print("数字7出现的次数：", count_seven)
if 99 not in nums:
    nums.append(99)
print("添加99后的列表:",nums)
nums.remove(7)
print("删除第一个7后的列表",nums)
sublist=nums[1:4]
print("第2-4元素组成的列表",sublist)