写一个登录注册代码

```python
import hashlib

users = {}

def register(username, password):
if username in users:
print("用户名已存在")
return False
else:
users[username] = hashlib.md5(password.encode()).hexdigest()
print("注册成功")
return True

def login(username, password):
if username not in users:
print("用户名不存在")
return False
elif users[username] != hashlib.md5(password.encode()).hexdigest():
print("密码错误")
return False
else:
print("登录成功")
return True

def main():
while True:
operation = input("请输入操作(1-注册，2-登录，3-退出):")
if operation == "1":
username = input("请输入用户名：")
password = input("请输入密码：")
register(username, password)
elif operation == "2":
username = input("请输入用户名：")
password = input("请输入密码：")
login(username, password)
elif operation == "3":
break
else:
print("输入错误，请重新输入")

if __name__ == "__main__":
main()
```