二分搜索 HDOJ 2675 Equation Again

题目传送门

 /*
     二分搜索：式子两边取对数，将x提出来，那么另一边就是一个常数了，函数是：lnx/x。二分搜索x，注意要两次
 */
 #include <cstdio>
 #include <algorithm>
 #include <cmath>
 using namespace std;

 const double e = exp (1.0);

 double cal(double x) {
     return log (x) / x;
 }

 int main(void)  {       //HDOJ 2675 Equation Again
     //freopen ("HDOJ_2675.in", "r", stdin);

     double y, k;
     while (scanf ("%lf", &y) == )  {
         k = ( + log (y)) / (e * y);
         //if (k > 1 / e)  {                 //囧！
         //    puts ("Happy to Women’s day!");    continue;
         //}
         if (k ==  / e) {
             printf ("%.5f\n", e); continue;
         }

         double x1, x2;
         double l = , r = e;
         for (int i=; i<=; ++i)  {
             double mid = (l + r) / ;
             if (cal (mid) >= k)   r = mid;
             else    l = mid;
         }
         x1 = l;
         l = e, r = 1e9;
         for (int i=; i<=; ++i)  {
             double mid = (l + r) / ;
             if (cal (mid) >= k)   l = mid;
             else    r = mid;
         }
         x2 = l;
         printf ("%.5f %.5f\n", x1, x2);
     }

     return ;
 }