HDU 2298 Toxophily

题目：http://acm.hdu.edu.cn/showproblem.php?pid=2298

竟然是物理题，初高中知识忘得差不多了,给出初速度，目标x,y位置，让你求射中该目标的角度.

三分+二分.斜抛的话,Y是先增大后减小的,三分求得斜抛的最大Y,若Y小于目标位置,则不可能射中.否则以此Y为上限进行二分.

嗯,好像也可以直接二分.

#include <iostream>
#include <cmath>
using namespace std;
double x,y,v;
const double g=9.8; 
const double PI=acos(-1.0);
double cal(double a)
{
    double y = x * tan(a) - g * x * x / 2 / ( v * v * cos(a) * cos(a));
    return y;
}
double twodivide(double low,double heigh)
{
    double mid;
    while(heigh-low>1e-9)
    {
        mid = (low + heigh)/2;
        if(cal(mid)>=y) heigh = mid;
        else low = mid;
    }
    return mid;
}

double threedivide(double low,double heigh)
{
    double left = low;
    double right = heigh;
    double mid;
    double midmid;
    while(right-left>1e-9)
    {
        mid = (left + right)/2;
        midmid = (mid + right)/2;
        if(cal(mid)>=cal(midmid)) right = midmid;
        else left = mid;
    }
    return mid;
}
int main(int argc, const char *argv[])
{
    int T;
    //freopen("input.txt","r",stdin);
    cin>>T;
    while(T--)
    {
        cin>>x>>y>>v;
        double maxy = threedivide(0,PI/2);
        if(cal(maxy)<y)
        {
            printf("-1
");
        }else 
        {
            printf("%.6lf
",twodivide(0,maxy));    
        }
    }
    return 0;
}