ARC059F

传送门

分析

见ptx大爷博客

代码

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<algorithm>
#include<cctype>
#include<cmath>
#include<cstdlib>
#include<ctime>
#include<queue>
#include<vector>
#include<set>
#include<map>
#include<stack>
using namespace std;
const int mod = 1e9+;
int dp[][],n,m;
char s[];
inline int pw(int x,int p){
    x%=mod;
    int res=;
    while(p){
      if(p&)res=(long long)res*x%mod;
      x=(long long)x*x%mod;
      p>>=;
    }
    return res;
}
int main(){
    int i,j,k;
    scanf("%d",&n);
    scanf("%s",s);
    m=strlen(s);
    dp[][]=;
    for(i=;i<=n;i++)
      for(j=;j<=i;j++){
          if(j)dp[i][j]=(dp[i][j]+dp[i-][j-]*)%mod;
          if(j==)dp[i][j]=(dp[i][j]+dp[i-][j])%mod;
          dp[i][j]=(dp[i][j]+dp[i-][j+])%mod;
      }
    int Ans=(long long)dp[n][m]*pw(pw(,m),mod-)%mod;
    printf("%d\n",Ans);
    return ;
}