51Nod 1013 3的幂的和（快速幂+逆元）

 #include <iostream>
 #include <algorithm>
 #include <string>

 #define MOD 1000000007
 using namespace std;

 long long quick_pow(long long a, long long b, long long c){
     long long ans = ;
     a = a%c;
     while (b > ){
         if (b %  == ){
             ans = (ans*a) % c;
         }
         b = b / ;
         a = (a*a) % c;
     }
     return ans;
 }

 int main(){
     ios::sync_with_stdio(false);
     /*
     freopen("in.txt", "r", stdin);
     freopen("out.txt", "w", stdout);
     */
     long long x;
     while (cin >> x){
         x++;
         long long ans = quick_pow(, x, MOD);
         ans--;
         ans = ans *  % MOD;
         cout << ans << endl;
     }
     //fclose(stdin);
     //fclose(stdout);
     //system("pause");
     return ;
 }