代码

// noip18e 填数游戏: closed form
// f(n,n) for n=1..8: 2,12,112,912,7136,56768,453504,3626752 ; f(n,m)=f(n,n)*3^(m-n) (m>=n)
#include <stdio.h>
typedef long long ll;
static const ll MOD = 1000000007;
static ll pw(ll b, ll e){ ll r=1; b%=MOD; while(e){ if(e&1) r=r*b%MOD; b=b*b%MOD; e>>=1; } return r; }
int main(){
    int n=0,m=0;
    if(scanf("%d %d",&n,&m)!=2) return 0;
    if(n>m){ int t=n; n=m; m=t; }
    static const ll f[9]={0,2,12,112,912,7136,56768,453504,3626752};
    ll ans;
    if(n==1) ans = pw(2,m);
    else ans = f[n]*pw(3,m-n)%MOD;
    printf("%lld\n",ans);
    return 0;
}

评测结果