hdu5879 Cure

题目链接：hdu5879 Cure

题解：用字符串输入。n很大时答案趋近与(π^2)/6。

 #include<cstdio>
 #include<algorithm>
 #include<iostream>
 #include<cstring>
 #include<queue>
 #include<cmath>
 using namespace std;
 const int N = ;
 const double pi = acos(-1.0);
 char a[N];
 double sum[N];

 int main(){
     int i, l, x;
     for(int i = ; i < N; ++i)
         sum[i] = sum[i - ] + 1.0/i/i;
     while(~scanf("%s",a)){
         l = strlen(a);
         if(l >= )printf("%.5lf\n",pi*pi/);
         else{
             for(x = i = ; i < l; ++i)
                 x =  * x + a[i] - '';
             printf("%.5lf\n", sum[x]);
         }
     }
     return ;
 }