[CF480C]Riding in a Lift

题目描述

DP

#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);
}