func main() {
	wg := sync.WaitGroup{}
	wg.Add(1)
	num := 28
	a := make(chan int)
	b := make(chan int)
	go func() {
		defer wg.Done()
		for i := 1; i < num; {
			<-a
			fmt.Println(i)
			i++
			fmt.Println(i)
			i++
			b <- 1

		}
	}()
	go func() {
		defer wg.Done()
		for i := 'a'; i < 'z'; {
			<-b
			fmt.Printf("%c", i)
			i++
			fmt.Printf("%c", i)
			i++
			a <- 1
		}
		<-b
	}()
	a <- 1
	wg.Wait()
	close(a)
	close(b)

输出 12AB34CD56EF78GH910IJ1112KL1314MN1516OP1718QR1920ST2122UV2324WX2526YZ

参考：Go语言面试题：交替打印数字和字母 - 知乎