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 = 1000010;
const double pi = acos(-1.0);
char a[N];
double sum[N];

int main(){
    int i, l, x;
    for(int i = 1; i < N; ++i)
        sum[i] = sum[i - 1] + 1.0/i/i;
    while(~scanf("%s",a)){
        l = strlen(a);
        if(l >= 7)printf("%.5lf\n",pi*pi/6);
        else{
            for(x = i = 0; i < l; ++i)
                x = 10 * x + a[i] - '0';
            printf("%.5lf\n", sum[x]);
        }
    }
    return 0;
}