Backend

package main



import (

  "fmt"

  "sync"

)



func joinChannels(chs ...<-chan int) <-chan int {

  mergedCh := make(chan int)



  go func() {

    wg := &sync.WaitGroup{}



    wg.Add(len(chs))



    for _, ch := range chs {

      go func(ch <-chan int, wg *sync.WaitGroup) {

        defer wg.Done()

        for id := range ch {

          mergedCh <- id

        }

      }(ch, wg)

    }



    wg.Wait()

    close(mergedCh)

  }()



  return mergedCh

}



func main() {



  a := make(chan int)

  b := make(chan int)

  c := make(chan int)



  go func() {

    for _, num := range []int{1, 2, 3} {

      a <- num

    }

    close(a)

  }()



  go func() {

    for _, num := range []int{20, 10, 30} {

      b <- num

    }

    close(b)

  }()



  go func() {

    for _, num := range []int{300, 200, 100} {

      c <- num

    }

    close(c)

  }()



  for num := range joinChannels(a, b, c) {

    fmt.Println(num)

  }



}