Codeforces Round #238 (Div. 1)__Toy Sum

const int MAXN = 1000010;

int vis[MAXN];
const int MID = 1000001;

int main()
{
//    freopen("in.txt", "r", stdin);
    int n, t;
    while (~RI(n))
    {
        CLR(vis, 0);
        REP(i, n)
        {
            RI(t);
            vis[t]++;
        }
        vector<int> ans;
        int need = 0;
        FE(i, 1, 1000000)
        {
            if (vis[i])
            {
                if (!vis[MID - i])
                    ans.push_back(MID - i);
                else
                    need++;
            }
        }
        need /= 2;

        int len = ans.size();
        REP(i, len)
            vis[ans[i]]++;
        for (int i = 1; need && i <= 1000000; i++)
        {
            if (!vis[i] && !vis[MID - i])
            {
                need--;
                ans.push_back(i);
                ans.push_back(MID - i);
            }
        }
        if (need)
            ans.push_back(1000000);
        len = ans.size();
        WI(len);
        REP(i, len)
        {
            if (i != 0)
                putchar(‘ ‘);
            printf("%d", ans[i]);
        }
        puts("");
    }
    return 0;
}