package main

import (
	"fmt"
)

func main() {
	n := 0
	fmt.Scan(&n)
	arr, tmp := make([]int, 1001), 0
	for i := 0; i < n; i++ {
		fmt.Scanf("%d", &tmp)
		arr[tmp]++
	}

	counter := 0
	res := make([]int, 0, n)
	for i, num := range arr {
		if num > 0 {
			res = append(res, i)
			counter++
		}
	}

	fmt.Println(counter)
	for i, num := range res {
		fmt.Printf("%d", num)
		if i == len(res)-1 {
			fmt.Printf("\n")
		} else {
			fmt.Printf(" ")
		}
	}
}

求教!