[CF480C]Riding in a Lift

题目描述

懒得写

DP

设f[i,j]表示第j次在i。
转移到的是个区间，用前缀和优化即可。

#include<cstdio>#include<algorithm>#include<cmath>#define fo(i,a,b) for(i=a;i<=b;i++)using namespace std;const int maxn=5000+10,mo=1000000007;int f[maxn*2][maxn];int i,j,k,l,t,n,m,a,b,ans;int main(){    scanf("%d%d%d%d",&n,&a,&b,&m);    f[a][0]=1;    f[a+1][0]=-1;    fo(j,0,m){        fo(i,1,n) (f[i][j]+=f[i-1][j])%=mo;        if (j==m) break;        fo(i,1,n)            if (i!=b&&f[i][j]){                k=abs(i-b)-1;                t=max(i-k,1);                (f[t][j+1]+=f[i][j])%=mo;                (f[i][j+1]-=f[i][j])%=mo;                t=min(i+k,n);                (f[i+1][j+1]+=f[i][j])%=mo;                (f[t+1][j+1]-=f[i][j])%=mo;            }    }    fo(i,1,n)        if (i!=b) (ans+=f[i][m])%=mo;    (ans+=mo)%=mo;    printf("%d\n",ans);}