URAL 1066 Garland 二分

二分H2的位置，判断条件为是否有Hi < 0

 #include <cstdio>
 #include <cstring>
 #include <cstdlib>
 #include <algorithm>
 #include <cmath>

 using namespace std;

 const double eps = 1e-;

 int N;
 double A;

 int dcmp( double a )
 {
     if ( fabs(a) < eps ) return ;
     return a <  ? - : ;
 }

 bool cal( double mid, double &ans )
 {
     double Hi2 = A, Hi1 = mid, Hi0;
     for ( int i = ; i < N - ; ++i )
     {
         Hi0 = 2.0 * ( Hi1 + 1.0 ) - Hi2;
         if ( dcmp( Hi0 ) <  ) return false;
         Hi2 = Hi1, Hi1 = Hi0;
     }
     ans = Hi0;
     return true;
 }

 int main()
 {
     while ( ~scanf( "%d%lf", &N, &A ) )
     {
         double l = 0.0, r = A;
         //int cnt = 0;
         double ans;
         while ( dcmp( r - l ) >  )
         {
             //++cnt;
             double mid = ( l + r ) / 2.0;
             //printf( "%f %f %f\n", l, r, mid );
             double tmp;
             if ( !cal( mid, tmp ) ) l = mid;
             else
             {
                 r = mid;
                 ans = tmp;
             }
             //if ( cnt > 100 ) break;
         }
         printf( "%.2f\n", ans );
     }
     return ;
 }