Watering Grass (贪心，最小覆盖)

参考：　　

https://blog.csdn.net/shuangde800/article/details/7828675

https://www.cnblogs.com/haoabcd2010/p/6171794.html?utm_source=itdadao&utm_medium=referral

#include <iostream>
#include <stdio.h>
#include <cstring>
#include <cmath>
#include <algorithm>


using namespace std;

struct node
{
    double L;
    double R; 
}a[10010];

bool cmp(node a, node b)
{
    return a.R > b.R;
}

void sort_node(int n)
{
    for (int i=0;i<n;i++)
    {
        int k=i;
        for (int j=i+1;j<n;j++)
        {
            if (a[j].R>a[k].R)
                k=j;
        }
        swap(a[k],a[i]);
    }
}

int main() 
{
    int n;
    double length, wide;　　　　// 都定义为double类型，因为使用勾股定理的时候肯定会算出小数
    double center, radius;     // center为圆心，radius为半径 
    double t;
    while(scanf("%d%lf%lf", &n, &length, &wide) != EOF)
    {
        for(int i = 0; i < n; ++i)
        {
            cin >> center >> radius;
            t = sqrt(radius*radius - (wide/2.0)*(wide/2.0));
            a[i].L = center - t;
            a[i].R = center + t;    
        }
        
        //sort(a, a+n, cmp);    快排不知道为什么不能AC 
        sort_node(n);
        
        double border = 0;
        int cnt = 0;
        while(border < length)　　// 此类型题目的固定套路
        {
            int i;
            for(i = 0; i < n; ++i)
            {
                if(a[i].L <= border && a[i].R > border)
                {
                    border = a[i].R;
                    cnt++;
                    break;
                }
            }
            
            if(i == n)    // 如果i==n，说明遍历完了这n个区间，仍然没有找到符合条件的，即不能完全覆盖 
                break;
        }
        
        if(border < length)
            cout << -1 << endl;
        else
            cout << cnt << endl;
    } 
            
    return 0;
}