406B

Solution

#include <cstdio>
using namespace std;

const int MAX_S = int(1e6) + 20;
const int s = int(1e6);

bool f[MAX_S];
int ans[MAX_S];
int ans_num;
int add_num;
bool first = true;
int n;

void output(int a)
{
    if (first)
    {
        first = false;
    }else
    {
        putchar(' ');
    }
    printf("%d", a);
}

int main()
{
    //input
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        int a;
        scanf("%d", &a);
        f[a] = true;
    }

    //work
    ans_num = 0;
    add_num = 0;
    for (int i = 1; i <= (s + 1) / 2; i++)
    {
        int a = s - i + 1;
        if (f[i] && f[a])
        {
            add_num++;
        }else if (f[i] || f[a])
        {
            if (f[i])
            {
                ans[ans_num++] = a;
            }else
            {
                ans[ans_num++] = i;
            }
        }
    }

    //output
    printf("%d\n", ans_num + add_num * 2);
    for (int i = 1; i <= (s + 1) / 2 && add_num; i++)
    {
        int a = s - i + 1;
        if (!f[i] && !f[a])
        {
            output(i);
            output(a);
            add_num--;
        }
    }

    for (int i = 0; i < ans_num; i++)
    {
        output(ans[i]);
    }
    return 0;
}