【HDOJ】4704 Sum

数学题。f(n) = 2^(n-1) mod (1e9+7)。

 #include <cstdio>

 #define MAXN 100005

 char buf[MAXN];
 __int64 phi = 1e9+;
 __int64 mod = 1e9+;

 __int64 power2(__int64 n) {
     __int64 ret = , base = ;

     --n;
     while (n) {
         if (n & )
             ret = ret * base % mod;
         base = base*base%mod;
         n >>= ;
     }
     return ret;
 }

 int main() {
     __int64 tmp;
     __int64 i, j;

     while (scanf("%s", buf) != EOF) {
         tmp = ;
         for (i=; buf[i]; ++i) {
             tmp = *tmp + (buf[i]-'');
             tmp %= phi;
         }
         tmp += phi;
         tmp = power2(tmp);
         printf("%I64d\n", tmp);
     }

     return ;
 }