贪心，区间覆盖问题 Watering Grass UVA - 10382

#include<iostream>
#include<cmath>
#include<algorithm>
using namespace std;
const int maxn = 10010;
double l, w;
pair<double, double>a[maxn];
int n;
int main(){
    while (cin >> n >> l >> w){
        int m = 0;
        for (int i = 0; i < n; ++i)
        {
            double x, r;
            cin >> x >> r;
            if (w>2 * r)continue;
            double temp = sqrt(r*r - w*w / 4);
            a[m++] = make_pair(x - temp, x + temp);        //计算区间
        }
        sort(a, a + m);
        double low = 0, up = 0;
        int cnt = 0;
        bool flag = 0;
        for (int i = 0; i < m; ++i)
        {
            if (a[i].first>up){ break; }        //保证新的区间与总的区间有交集
            if (a[i].second > up){
                for (int j = i; j < m&&a[j].first <= low;++j)
                if (a[j].second>up){
                    up = a[j].second;        //更新总的区间
                }
                cnt++;
                if (up >= l){
                    flag = 1;
                    break;
                }
                low = up;
            }
        }
        cout << (flag ? cnt : -1) << endl;
    }
}