Gosched

Add code in line A to assure that the lowercase letters and capital letters are printed consecutively.

package main
import (
    "fmt"
    "runtime"
    "sync"
)
const N = 26
func main() {
    const GOMAXPROCS = 1
    runtime.GOMAXPROCS(GOMAXPROCS)
    var wg sync.WaitGroup
    wg.Add(2 * N)
    for i := 0; i < N; i++ {
        go func(i int) {
            defer wg.Done()
            // A
            fmt.Printf("%c", 'a'+i)
        }(i)
        go func(i int) {
            defer wg.Done()
            fmt.Printf("%c", 'A'+i)
        }(i)
    }
    wg.Wait()
}

Answer

package main
import (
    "fmt"
    "runtime"
    "sync"
)
const N = 26
func main() {
    const GOMAXPROCS = 1
    runtime.GOMAXPROCS(GOMAXPROCS)
    var wg sync.WaitGroup
    wg.Add(2 * N)
    for i := 0; i < N; i++ {
        go func(i int) {
            defer wg.Done()
            runtime.Gosched()
            fmt.Printf("%c", 'a'+i)
        }(i)
        go func(i int) {
            defer wg.Done()
            fmt.Printf("%c", 'A'+i)
        }(i)
    }
    wg.Wait()
}