51nod 1043 幸运号码

#include <bits/stdc++.h>using namespace std;const long long mod=1e9+7;long long fir[10100],sec[10100];int main(){long long n,i,j,k,ans;while(cin>>n){memset(fir,0,sizeof(fir));memset(sec,0,sizeof(sec));for(i=1;i<=9;i++)fir[i]=1;for(i=0;i<=9;i++)sec[i]=1; for(i=2;i<=n;i++){for(j=i*9;j>=0;j--){for(k=1;k<=9&&j-k>=0;k++){fir[j]+=fir[j-k];sec[j]+=sec[j-k];} fir[j]%=mod;sec[j]%=mod;}}ans=0;for(i=1;i<=9*n;i++)ans=(ans+fir[i]*sec[i])%mod;printf("%lld\n",ans);}}