List numbers of 4 and 5 digits that one is the double of the other and all use the 9 digits from 1 to 9 without repiting.

digits

package main

import "fmt"

func main() {
	for j := 6234; j < 9876; j++ {
		i := 2 * j
		if nineChars(i, j) {
			fmt.Printf("%d\t%d\n", i, j)
		}
	}
}

func nineChars(i, j int) bool {
	str := fmt.Sprintf("%d%d", i, j)
	digit := [9]bool{}
	for _, c := range str {
		if c == '0' {
			return false
		}
		if digit[c-'1'] {
			return false
		}
		digit[c-'1'] = true
	}
	return true
}

https://play.golang.org/p/_h14fwBD0xY