Suppose you are given the following code:

class FooBar {public void foo() {for (int i = 0; i < n; i++) {print("foo");}}public void bar() {for (int i = 0; i < n; i++) {print("bar");}}
}

The same instance of FooBar will be passed to two different threads:

thread A will call foo(), while
thread B will call bar().

Modify the given program to output "foobar" n times.

Example 1:

Input: n = 1
Output: "foobar"
Explanation: There are two threads being fired asynchronously. One of them calls foo(), while the other calls bar().
"foobar" is being output 1 time.

Example 2:

Input: n = 2
Output: "foobarfoobar"
Explanation: "foobar" is being output 2 times.

Constraints:

1 <= n <= 1000

The requirement:

Two threads:
- Thread A calls foo()
- Thread B calls bar()
Output must be:

foobarfoobarfoobar...

The key problem is: how do we make one thread wait until it is its turn?

Solution 1: Use `Semaphore` (Recommended)

Use two semaphores:

foo_sem: controls when foo() can run
bar_sem: controls when bar() can run

Initial state:

foo_sem = 1   # foo can run first
bar_sem = 0   # bar must wait

Flow:

foo thread:print fooallow barbar thread:wait for permissionprint barallow foo

Python Implementation

from threading import Semaphoreclass FooBar:def __init__(self, n: int):self.n = nself.foo_sem = Semaphore(1)self.bar_sem = Semaphore(0)def foo(self, printFoo) -> None:for _ in range(self.n):self.foo_sem.acquire()printFoo()self.bar_sem.release()def bar(self, printBar) -> None:for _ in range(self.n):self.bar_sem.acquire()printBar()self.foo_sem.release()

Execution example

Initial:

foo_sem = 1
bar_sem = 0

First iteration

Foo thread:

acquire foo_sem↓
print foo↓
release bar_sem

State:

foo_sem = 0
bar_sem = 1

Bar thread:

acquire bar_sem↓
print bar↓
release foo_sem

State:

foo_sem = 1
bar_sem = 0

Repeat.

Output:

foo
bar
foo
bar
foo
bar

Solution 2: Use Condition

You can also solve it using Condition.

Maintain a shared variable:

turn = "foo"

Foo waits until:

turn == "foo"

Bar waits until:

turn == "bar"

Python Implementation

from threading import Conditionclass FooBar:def __init__(self, n: int):self.n = nself.condition = Condition()self.foo_turn = Truedef foo(self, printFoo) -> None:for _ in range(self.n):with self.condition:while not self.foo_turn:self.condition.wait()printFoo()self.foo_turn = Falseself.condition.notify()def bar(self, printBar) -> None:for _ in range(self.n):with self.condition:while self.foo_turn:self.condition.wait()printBar()self.foo_turn = Trueself.condition.notify()