题解 | #序列操作#

import java.util.HashSet;
import java.util.Scanner;
import java.util.Stack;

public class Main {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);

        while(in.hasNext()){
            // solution1(in);
            solution2(in);
        }
    }

    /**
     * 模拟法: 数组
     * @param in
     */
    private static void solution1(Scanner in){
        int n = in.nextInt();
        int m = in.nextInt();

        int[] nums = new int[n+1];
        int[] moves = new int[m+1];
        for(int i=1; i<=m; i++){
            moves[i] = in.nextInt();
        }

        for(int i=m; i>=1; i--){
            int index = moves[i];
            if(nums[index] == 0){
                System.out.println(index);
                nums[index] = -1;
            }
        }

        for(int i=1; i<=n; i++){
            if(nums[i] == 0){
                System.out.println(i);
            }
        }
    }

    /**
     * 模拟法: set + stack
     * @param in
     */
    private static void solution2(Scanner in){
        int n = in.nextInt();
        int m = in.nextInt();

        HashSet<Integer> pushSet = new HashSet<>();
        HashSet<Integer> popSet = new HashSet<>();
        Stack<Integer> stack = new Stack<>();
        int num;
        for(int i=1; i<=m; i++){
            num = in.nextInt();
            pushSet.add(num);
            stack.add(num);
        }

        int top;
        while(!stack.isEmpty()){
            top = stack.pop();
            if(!popSet.contains(top)){
                System.out.println(top);
                popSet.add(top);
            }
        }

        for(int i=1; i<=n; i++){
            if(!pushSet.contains(i)){
                System.out.println(i);
            }
        }
    }
}