HDOJ 2899 二分搜索

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=2899

解题思路：F(x)求导，二分搜索F'(x)=0时的x的值。

#include <iostream>
#include <cmath>
using namespace std;
const double eps = 1e-12;

double bsearch(double y)
{
    double mid, tmp;
    double left = 0, right = 100;
    while (fabs(right - left) > eps)
    {
        mid = (left + right) / 2;
        tmp = 42 * pow(mid, 6) + 48 * pow(mid, 5) + 21 * pow(mid, 2) + 10 * mid;
        if (fabs(tmp - y) <= eps) return mid;
        if (tmp < y) left = mid + eps;
        else right = mid - eps;
    }
    return (left + right) / 2;
}

int main()
{
    int t;
    double x, y;
    for (scanf("%d", &t); t--; )
    {
        scanf("%lf", &y);
        x = bsearch(y);
        printf("%.4lf\n", 6 * pow(x, 7) + 8 * pow(x, 6) + 7 * pow(x, 3) + 5 * pow(x, 2) - x * y);
    }
}