题目:
题解:
f(i,j,0/1)表示划分了i段,以第j个结尾,最后一段是上升0\下降1的方案数
f(i,j,0)=∑k=i+1j−1∑y(k)<y(j)f(i,k,0)+∑k=i+1j−1∑y(k)<y(j)f(i−1,k,1)
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f(i,j,1)=∑k=i+1j−1∑y(k)>y(j)f(i−1,k,0)+∑k=i+1j−1∑y(k)>y(j)f(i,k,1)
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两个限制条件,第一个边做边维护,第二个用bit维护前缀和,01分开维护,滚动数组
然后就是O(nlogn)了
代码:
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int mod=100007;
const int N=50005;
const int inf=100000;
struct hh{int x,y;}p[N];
int cmp(hh a,hh b){return a.x<b.x;}
int f[12][N][2],c[inf+5][2][2];
void add(int loc,int v,int id,int jd)
{
for (int i=loc;i<=inf;i+=i&(-i))
c[i][id][jd]=(c[i][id][jd]+v)%mod;
}
int qurry(int loc,int id,int jd)
{
int ans=0;
for (int i=loc;i>=1;i-=i&(-i))
ans=(ans+c[i][id][jd])%mod;
return ans;
}
int main()
{
freopen("line.in","r",stdin);
freopen("line.out","w",stdout);
int n,k;scanf("%d%d",&n,&k);
for (int i=1;i<=n;i++) scanf("%d%d",&p[i].x,&p[i].y);
sort(p+1,p+n+1,cmp);
for (int i=1;i<=n;i++) f[0][i][0]=f[0][i][1]=1;
for (int i=1;i<=k;i++)
{
memset(c,0,sizeof(c));
add(p[i].y,f[i-1][i][0],(i-1)&1,0);
add(p[i].y,f[i-1][i][1],(i-1)&1,1);
for (int j=i+1;j<=n;j++)
{
f[i][j][0]=(f[i][j][0]+qurry(p[j].y-1,i&1,0)+qurry(p[j].y-1,(i-1)&1,1))%mod;
f[i][j][1]=(f[i][j][1]+qurry(inf,(i-1)&1,0)-qurry(p[j].y-1,(i-1)&1,0)+qurry(inf,i&1,1)-qurry(p[j].y,i&1,1)+mod)%mod;
//注意这里一减可能是负数,要+mod
add(p[j].y,f[i][j][0],i&1,0);
add(p[j].y,f[i-1][j][0],(i-1)&1,0);
add(p[j].y,f[i][j][1],i&1,1);
add(p[j].y,f[i-1][j][1],(i-1)&1,1);
}
}
int ans=0;
for (int i=1;i<=n;i++) ans=(ans+f[k][i][0]+f[k][i][1])%mod;
printf("%d",ans);
}