Final Exam Arrangement

const int MAXN = 1100000;

struct Node
{
    int v, id, isr;
    int operator< (const Node& rhs) const
    {
        if (v != rhs.v)
            return v < rhs.v;
        else
            return isr > rhs.isr;
    }
} ipt[MAXN];

VI ans[MAXN];
int vis[MAXN];

int main()
{
    int n, l, r;
    while (~RI(n))
    {
        CLR(vis, 0);
        REP(i, MAXN)
            ans[i].clear();
        REP(i, n)
        {
            RII(l, r);
            int x = i << 1, y = x | 1;
            ipt[x].isr = 0;
            ipt[x].v = l;
            ipt[y].isr = 1;
            ipt[y].v = r;
            ipt[x].id = ipt[y].id = i + 1;
        }
        n <<= 1;
        sort(ipt, ipt + n);
        int val = 0;
        REP(i, n)
        {
            if (ipt[i].isr)
            {
                if (vis[ipt[i].id] != val)
                    continue;
                val++;
            }
            else
            {
                ans[val].push_back(ipt[i].id);
                vis[ipt[i].id] = val;
            }
        }
        WI(val);
        REP(i, val) REP(j, ans[i].size())
            printf("%d%c", ans[i][j], j == (int)ans[i].size() - 1 ? '\n' : ' ');
    }
    return 0;
}