Challenge 3 — Solution

Task: Write a function worker(id int, wg *sync.WaitGroup) that
prints "Worker started" and "Worker finished" with the id, using
defer wg.Done(). Launch 4 workers using a sync.WaitGroup, then print
"All done" only after wg.Wait() returns.

package main

import (
	"fmt"
	"sync"
)

func worker(id int, wg *sync.WaitGroup) {
	defer wg.Done()
	fmt.Println("Worker", id, "started")
	fmt.Println("Worker", id, "finished")
}

func main() {
	var wg sync.WaitGroup

	for i := 1; i <= 4; i++ {
		wg.Add(1)
		go worker(i, &wg)
	}

	wg.Wait()
	fmt.Println("All done")
}


Expected output (worker order may vary, but "All done" is always
last):

Worker 2 started
Worker 2 finished
Worker 1 started
Worker 4 started
Worker 4 finished
Worker 1 finished
Worker 3 started
Worker 3 finished
All done

Notes:
- wg.Add(1) is called once per worker BEFORE launching it, so the
  WaitGroup's internal counter correctly reaches 4 before any
  worker has a chance to call Done().
- defer wg.Done() guarantees Done() runs when worker() returns,
  which is what eventually brings the counter back down to 0.
- wg.Wait() blocks main() until that counter hits 0 — this is why
  "All done" is guaranteed to print last, even though the order of
  the workers' own output is unpredictable.