uva 10382 Watering Grass （最少区间覆盖）

首先，直径小于草坪宽度的喷水器不选。

然后，按喷水器能够覆盖草坪的矩形范围 转换成 区间覆盖问题。

#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
#include <string>
#include <stack>
#include <cmath>
#include <queue>
#include <set>
#include <map>
typedef long long ll;
using namespace std;

const int inf=0x3f3f3f3f;
const int maxn=1e4+5;

int n,len,wid;
int num;

struct water{
	double l,r;//区间左右坐标
};
water wa[maxn];

bool cmp(water a,water b){
	return a.l<b.l;
}

int solve(){
	double start=0;
	int i=0,ans=0;
	while(i<num){
		if(wa[i].l >start)return -1;
		double maxr=-1;
		while(i<num && wa[i].l<=start ){
			maxr=max(maxr,wa[i].r);
			i++;
		}
		start=maxr;
		ans++;
	}
	if(start<len)return -1;
	return ans;
}

int main()
{
	while(scanf("%d %d %d",&n,&len,&wid)!=EOF){
		int pos,radius;
		num=0;
		for(int i=0;i<n;++i){
			scanf("%d %d",&pos,&radius);
			if(2*radius>=wid){
				double temp=sqrt( ((double)radius)*radius-((double)wid)*wid/4);
				//printf("%f
", temp);
				wa[num].l=pos-temp;
				wa[num].r=pos+temp;
				//printf("l==%f r==%f
",wa[num].l,wa[num].r );
				num++;
			}
		}
		sort(wa,wa+num,cmp);//按左坐标从小到大排序

		int ans=solve();

		printf("%d
",ans );
	}
	return 0;
}