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-9;

int N;
double A;

int dcmp( double a )
{
    if ( fabs(a) < eps ) return 0;
    return a < 0 ? -1 : 1;
}

bool cal( double mid, double &ans )
{
    double Hi2 = A, Hi1 = mid, Hi0;
    for ( int i = 0; i < N - 2; ++i )
    {
        Hi0 = 2.0 * ( Hi1 + 1.0 ) - Hi2;
        if ( dcmp( Hi0 ) < 0 ) 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 ) > 0 )
        {
            //++cnt;
            double mid = ( l + r ) / 2.0;
            //printf( "%f %f %f
", l, r, mid );
            double tmp;
            if ( !cal( mid, tmp ) ) l = mid;
            else
            {
                r = mid;
                ans = tmp;
            }
            //if ( cnt > 100 ) break;
        }
        printf( "%.2f
", ans );
    }
    return 0;
}