1、输入小岛坐标后更新雷达坐标。再将雷达区间按右端点从小到大排序
2、区间选点

905. 区间选点（右端点排序）

#include <iostream>
#include <algorithm>
#include <cmath>

using namespace std;

const int N = 1010;

struct Range
{
    double l, r;

    bool operator < (const Range& t)
    {
        return r < t.r; //按右端点升序
    }
}range[N];

int n, d;

int main()
{
    cin >> n >> d;
    bool flag = true;

    for (int i = 0; i < n; i++)
    {
        int x, y;
        cin >> x >> y;
        if (y > d) //不会有雷达能检测到
        {
            flag = false;
            break;
        }
        double l = x - sqrt(d*d - y*y);
        double r = x + sqrt(d*d - y*y);
        range[i] = {l, r};
    }


    if (!flag)
    {
        puts("-1");
        return 0;
    } 
    sort(range, range + n);

    //枚举每段区间
    double maxt = -0x3f3f3f3f, res = 0; //maxt:上一段的右端点
    for (int i = 0; i < n; i++)
        if (range[i].l > maxt + 1e-9) //若不能有那一段区间有交集，就更新
        {
            maxt = range[i].r;
            res++;
        }

    cout <<res;

    return 0;
}