Submission #1106729

#	Time	Username	Problem	Language	Result	Execution time	Memory
1106729		12345678	Boat (APIO16_boat)	C++17	9 / 100	2059 ms	12416 KiB

boat

#include <bits/stdc++.h>

using namespace std;

#define ll long long

const int nx=505, mod=1e9+7;
ll n, a[nx], b[nx], dp[nx][2*nx], qs[nx][2*nx], sz[2*nx], t, pv, ncr[nx][nx], choose[2*nx][nx], l[nx], r[nx];
map<ll, ll> mp;

ll invmod(ll a, ll x)
{
    if (x==0) return 1;
    auto tmp=invmod(a, x/2);
    tmp=(tmp*tmp)%mod;
    if (x%2) return (tmp*a)%mod;
    else return tmp;
}

int main()
{
    cin.tie(NULL)->sync_with_stdio(false);
    cin>>n;
    mp[0]=0;
    for (int i=1; i<=n; i++) cin>>a[i]>>b[i], mp[--a[i]]=0, mp[b[i]]=0;
    for (auto &[x, y]:mp) y=t, sz[t]=x-pv, pv=x, t++;
    for (int i=1; i<=n; i++) l[i]=mp[a[i]], r[i]=mp[b[i]];
    ncr[0][0]=1;
    for (int i=1; i<=n; i++)
    {
        for (int j=0; j<=i; j++)
        {
            ncr[i][j]=ncr[i-1][j];
            if (j>0) ncr[i][j]=(ncr[i][j]+ncr[i-1][j-1])%mod;
        }
    }
    for (int i=1; i<t; i++)
    {
        //cout<<"sz "<<i<<' '<<sz[i]<<'\n';
        ll tmp=1;
        choose[i][0]=tmp;
        for (int j=1; j<=min(sz[i], n); j++) tmp=(((tmp*(sz[i]-j+1))%mod)*invmod(j, mod-2))%mod, choose[i][j]=tmp;
    }
    for (int i=1; i<=n; i++)
    {
        //cout<<"l "<<i<<' '<<l[i]<<' '<<r[i]<<'\n';
        for (int j=1; j<t; j++)
        {
            if (l[i]<j&&j<=r[i])
            {
                dp[i][j]=((qs[i-1][j-1]+1)*sz[j])%mod;
                ll cnt=1;
                for (int k=i-1; k>=1; k--)
                {
                    if (l[k]<j&&j<=r[k]) cnt++;
                    //if (i==n) cout<<"debug "<<k<<' '<<cnt<<'\n';
                    if (cnt>=2)
                    {
                        ll sm=0;
                        for (int tmp=0; tmp<=cnt-2; tmp++) sm=(sm+choose[j][tmp+2]*ncr[cnt-2][tmp])%mod;
                        //if (i==n) cout<<"sm "<<sm<<'\n', cout<<"choose "<<choose[1][0]<<' '<<ncr[1][0]<<'\n';
                        dp[i][j]=(dp[i][j]+sm*(qs[k-1][j-1]+1))%mod;
                    }
                }
            }
            //cout<<"dp "<<i<<' '<<j<<' '<<dp[i][j]<<'\n';
            qs[i][j]=((dp[i][j]+qs[i-1][j]+qs[i][j-1]-qs[i-1][j-1])%mod+mod)%mod;
        }
    }
    cout<<qs[n][t-1];
}

/*
2
1 3
1 3
*/

• Subtask 1: 9 / 9
• Subtask 2: 0 / 22
• Subtask 3: 0 / 27
• Subtask 4: 0 / 42