Suppose we have a class:

public class Foo {
  public void first() { print("first"); }
  public void second() { print("second"); }
  public void third() { print("third"); }
}

The same instance of Foo will be passed to three different threads. Thread A will call first(), thread B will call second(), and thread C will call third(). Design a mechanism and modify the program to ensure that second() is executed after first(), and third() is executed after second().

Note:

We do not know how the threads will be scheduled in the operating system, even though the numbers in the input seem to imply the ordering. The input format you see is mainly to ensure our tests' comprehensiveness.

Example 1:

Input: nums = [1,2,3]
Output: "firstsecondthird"
Explanation: There are three threads being fired asynchronously. The input [1,2,3] means thread A calls first(), thread B calls second(), and thread C calls third(). "firstsecondthird" is the correct output.

Example 2:

Input: nums = [1,3,2]
Output: "firstsecondthird"
Explanation: The input [1,3,2] means thread A calls first(), thread B calls third(), and thread C calls second(). "firstsecondthird" is the correct output. 

Constraints:

• nums is a permutation of [1, 2, 3].

Solution 1: Lock (Most Common)

The idea is simple:

• first() runs immediately.
• second() waits until first() finishes.
• third() waits until second() finishes.

from threading import Lock

class Foo:
    def __init__(self):
        self.lock2 = Lock()
        self.lock3 = Lock()

        self.lock2.acquire()
        self.lock3.acquire()

    def first(self, printFirst: 'Callable[[], None]') -> None:
        printFirst()
        self.lock2.release()

    def second(self, printSecond: 'Callable[[], None]') -> None:
        with self.lock2:
            printSecond()
            self.lock3.release()

    def third(self, printThird: 'Callable[[], None]') -> None:
        with self.lock3:
            printThird()

Solution 2: Semaphore (Very Common)

This is probably the cleanest solution.

Why Semaphore(0)?

Initial permits:

Calling

blocks until someone calls

So

Solution 3: Condition Variable

Here:

• stage == 1 → first should run
• stage == 2 → second should run
• stage == 3 → third should run

Always use a while loop instead of if around wait() to handle spurious wakeups.

Solution 4: Event (Simplest)

Python's Event is designed exactly for "wait until something happens."

An Event behaves like a boolean flag:

• Initially False.
• wait() blocks until the flag becomes True.
• set() changes the flag to True and wakes all waiting threads.

Interview rule of thumb

When designing concurrent systems:

• Need mutual exclusion?
  → Lock
• Need N threads/resources at the same time?
  → Semaphore
• Need to wait for a condition/state change?
  → Condition
• Need one thread to broadcast "I am ready/done/stopped"?
  → Event

A good mental model: