P2467 [SDOI2010]地精部落

 

 

  bug: 前缀和不要写错啊啊， 比如 s[r]-s[l-1]

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#include <iostream> #include <cstring> #include <algorithm> using namespace std; const int N=4202; int mod ; int f[N][N],s[N][N],n;    void sov(){     int i,j;     f[1][1]=s[1][1]=1;           for(i=2;i<=n;i++){         for(j=1;j<=i;j++)         if(i&1){                 f[i][j]=(s[i-1][i-1]-s[i-1][j-1]+mod)%mod;                 s[i][j]=s[i][j-1]+f[i][j],                 s[i][j]%=mod;         }           else{                 f[i][j]=s[i-1][j-1];                 s[i][j]=s[i][j-1]+f[i][j],                 s[i][j]%=mod;         }     }           cout<<(2*s[n][n])%mod<<endl;  }  signed main(){     cin>>n>>mod;     sov();  }