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[];

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

 void sort_node(int n)
 {
     for (int i=;i<n;i++)
     {
         int k=i;
         for (int j=i+;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 = ; 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 = ;
         int cnt = ;
         while(border < length)　　// 此类型题目的固定套路
         {
             int i;
             for(i = ; 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 << - << endl;
         else
             cout << cnt << endl;
     } 

     return ;
 }