我要评论
思路：双指针思路可以吗，我就直接找g,如果后一个是1就cnt++，如果不是数字，用一个指针i指向g，另一个指针j移动，当不是g时停止，统计g的个数，如果是奇数个同时g的下一个是1，cnt++，如果为偶数同时g的下一个是1，就用那唯一一次机会g的国特抵消一次，cnt++，此后不再对偶数个g进行cnt++,同时把i移动到j指向g位置,就这样一直遍历字符串；
s = input()
cnt = 0
king_used = false
i = 0
while i < len(s):
if s[i] == 'g':
if i + 1 < len(s):
if s[i + 1] == '1':
cnt += 1
i += 2
continue
elif s[i + 1] in ['2', '3']:
i += 2
continue
else:
j = i + 1
while j < len(s) and s[j] == 'g':
j += 1
g_count = j - i
if g_count % 2 == 1:
cnt += 1
elif g_count % 2 == 0 and not king_used:
cnt += 1
king_used = true
i = j
else:
i += 1
else:
i += 1
print(cnt)
转c++我不知道为啥我c++代码过不了，但是python嫩果
一、mysql 基础语句
1. 数据库操作 创建数据库
create database school_db;
删除数据库
drop database school_db;
选择数据库
use school_db;
2. 表操作 创建表
create table students (
student_id int auto_increment primary key,
name varchar(50) not null,
age int,
gender enum('男', '女'),
enrollment_date date default current_date
);删除表
drop table students;
修改表结构
alter table students add column email varchar(100); alter table students modify column age tinyint; alter table students drop column gender;
3. crud 操作
插入数据
insert into students (name, age, gender)
values ('张三', 18, '男'), ('李四', 19, '女');查询数据
-- 基本查询 select * from students; -- 条件查询 select name, age from students where age > 18; -- 排序 select * from students order by age desc; -- 分组 select gender, count(*) from students group by gender; -- 分页 select * from students limit 5 offset 0; -- 第一页，每页5条
更新数据
update students set age = 20 where name = '张三';
删除数据
delete from students where student_id = 1;
二、外键与关联查询
1. 外键概念
外键(foreign key)用于建立表与表之间的关联关系，确保数据完整性。
2. 创建带外键的表
-- 班级表
create table classes (
class_id int auto_increment primary key,
class_name varchar(50) not null,
teacher varchar(50)
);
-- 学生表(带外键)
create table students (
student_id int auto_increment primary key,
name varchar(50) not null,
age int,
class_id int,
foreign key (class_id) references classes(class_id)
on delete set null
on update cascade
);3. 外键约束选项
on delete cascade
: 主表记录删除时，从表相关记录自动删除
on delete set null
: 主表记录删除时，从表外键设为null
on delete restrict
: 拒绝删除主表记录(默认)
on update cascade
: 主表主键更新时，从表外键同步更新
4. 关联查询
内连接(inner join)
select s.name, s.age, c.class_name from students s inner join classes c on s.class_id = c.class_id;
左连接(left join)
select s.name, c.class_name from students s left join classes c on s.class_id = c.class_id;
右连接(right join)
select s.name, c.class_name from students s right join classes c on s.class_id = c.class_id;
全连接(full join) - mysql通过union实现
select s.name, c.class_name from students s left join classes c on s.class_id = c.class_id union select s.name, c.class_name from students s right join classes c on s.class_id = c.class_id;
三、完整示例
1. 创建数据库和表
-- 创建数据库
create database school_management;
use school_management;
-- 创建班级表
create table classes (
class_id int auto_increment primary key,
class_name varchar(50) not null,
teacher varchar(50),
created_at timestamp default current_timestamp
);
-- 创建学生表(带外键)
create table students (
student_id int auto_increment primary key,
name varchar(50) not null,
age int check (age between 10 and 30),
gender enum('男', '女'),
class_id int,
foreign key (class_id) references classes(class_id)
on delete set null
on update cascade,
created_at timestamp default current_timestamp
);
-- 创建成绩表(多外键)
create table scores (
score_id int auto_increment primary key,
student_id int,
subject varchar(50),
score decimal(5,2),
exam_date date,
foreign key (student_id) references students(student_id)
on delete cascade
on update cascade
);2. 插入测试数据
-- 插入班级数据
insert into classes (class_name, teacher) values
('一年级一班', '王老师'),
('一年级二班', '李老师'),
('二年级一班', '张老师');
-- 插入学生数据
insert into students (name, age, gender, class_id) values
('张三', 18, '男', 1),
('李四', 19, '女', 1),
('王五', 17, '男', 2),
('赵六', 20, '女', null);
-- 插入成绩数据
insert into scores (student_id, subject, score, exam_date) values
(1, '数学', 90.5, '2023-06-15'),
(1, '语文', 85.0, '2023-06-16'),
(2, '数学', 92.0, '2023-06-15'),
(3, '英语', 88.5, '2023-06-17');3. 复杂查询示例
查询每个班级的学生人数
select c.class_name, count(s.student_id) as student_count from classes c left join students s on c.class_id = s.class_id group by c.class_id;
查询学生及其班级和成绩(多表连接)
select s.name, s.age, c.class_name, sc.subject, sc.score from students s left join classes c on s.class_id = c.class_id left join scores sc on s.student_id = sc.student_id order by s.name, sc.subject;
查询平均分高于85的班级
select c.class_name, avg(sc.score) as avg_score from classes c join students s on c.class_id = s.class_id join scores sc on s.student_id = sc.student_id group by c.class_id having avg_score > 85;
使用子查询
-- 查询没有参加任何考试的学生 select name from students where student_id not in (select distinct student_id from scores);
四、外键管理
1. 查看外键约束
select
table_name, column_name, constraint_name,
referenced_table_name, referenced_column_name
from
information_schema.key_column_usage
where
referenced_table_schema = 'school_management';2. 删除外键约束
alter table students drop foreign key students_ibfk_1;
3. 添加外键约束
alter table students add constraint fk_class foreign key (class_id) references classes(class_id) on delete set null on update cascade;
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