poj 1957 二分搜索

#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <math.h>
#include <cstring>
using namespace std;
#define maxa 1000
#define maxn 1000+16
typedef long long ll;
int n;
double a, b, h[maxn];
bool c(double mid)
{
    h[1] = mid;
    for (int i = 2; i < n; i++)
    {
        h[i] = 2 * h[i - 1] + 2 - h[i - 2];
        if (h[i] < 0) return false;
    }
    b = h[n - 1];
    return true;
}
int main()
{
    cin >> n >> a;
    h[0] = a;
    double lb = -1, ub = maxa + 16;
    for (int i = 0; i < 100; i++)
    {
        double mid = (lb + ub) / 2;
        if (c(mid))  ub = mid;
        else lb = mid;
    }
    printf("%.2lf\n", b);
    return 0;
}