Ideas
交替锁的设计，两把锁，foo执行的时候把foo lock acquire，print完了只有把bar lock release，这样foo就得等着，然后bar执行的时候把bar lock acquire，print完了之后把foo lock release，这样就能交替打印foo和bar了。
Code
Python
from threading import Lock
class FooBar:
def __init__(self, n):
self.n = n
self.print_foo = Lock()
self.print_bar = Lock()
self.print_bar.acquire()
def foo(self, printFoo: 'Callable[[], None]') -> None:
for i in range(self.n):
self.print_foo.acquire()
# printFoo() outputs "foo". Do not change or remove this line.
printFoo()
self.print_bar.release()
def bar(self, printBar: 'Callable[[], None]') -> None:
for i in range(self.n):
self.print_bar.acquire()
# printBar() outputs "bar". Do not change or remove this line.
printBar()
self.print_foo.release()
