bzoj1925地精部落——数学

Posted 2020-11-10 Zinn

题目：https://www.lydsy.com/JudgeOnline/problem.php?id=1925

真是精妙的递推式。。。好难想到啊；

详见这位的博客：https://www.cnblogs.com/Narh/p/9135070.html

代码如下：

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int n,p,f[3][4200],ans;
int main()
{
    scanf("%d%d",&n,&p);
    f[0][1]=1;
    for(int i=1;i<=n;i++)
        for(int j=1;j<=i;j++)
            f[i%2][j]=(f[i%2][j-1]+f[(i+1)%2][i-j+1])%p;
    for(int i=1;i<=n;i++)
        (ans+=f[n%2][i])%=p;
    (ans<<=1)%=p;
    printf("%d",ans);
    return 0;
}