Uva 10382 (区间覆盖) Watering Grass

和 Uva 10020几乎是一样的，不过这里要把圆形区域转化为能够覆盖的长条形区域(一个小小的勾股定理)

学习一下别人的代码，练习使用STL的vector容器

这里有个小技巧，用一个微小量EPS来弥补浮点运算中的误差

//#define LOCAL
#include <vector>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <functional>
using namespace std;

const int MAXN = 10240;
const double EPS = 1e-11;

struct Range
{
    double a, b;
    inline bool operator< (const Range& rhs) const
    {
        return a < rhs.a || (a == rhs.a && b < rhs.b);
    }
};

int main(void)
{
    #ifdef LOCAL
        freopen("10382in.txt", "r", stdin);
    #endif

    int n, l, w;
    double lenth, width;
    vector<Range> ranges;
    ranges.reserve(MAXN);//reserve()函数提前设定容量大小，避免多次容量扩充操作导致效率低下

    while(scanf("%d%d%d", &n, &l, &w) == 3)
    {
        lenth = (double)l;
        width = w / 2.0;
        ranges.clear();

        for(int i = 0; i < n; ++i)
        {
            int position, radius;
            double xw;
            Range range;

            scanf("%d%d", &position, &radius);
            if(radius * 2 <= w)    continue;
            xw = sqrt((double)radius * radius - width * width);

            range.a = position - xw;
            if(range.a > lenth + EPS)    continue;
            else if(range.a - EPS < 0.0)    range.a = 0.0;
            range.b = position + xw;
            ranges.push_back(range);
        }

        sort(ranges.begin(), ranges.end());

        int minCover = 0;
        double start = 0.0, end = 0.0;
        for(vector<Range>::iterator pr = ranges.begin(); pr != ranges.end(); )
        {
            start = end;
            if(pr->a > start + EPS)
            {
                minCover = -1;
                break;
            }
            ++minCover;
            while(pr != ranges.end() && pr->a <= start)
            {
                if(pr->b > end + EPS)    end = pr->b;
                ++pr;
            }
            if(end > lenth + EPS)
                break;
        }
        if(end + EPS < lenth)    minCover = -1;
        printf("%d
", minCover);
    }
    return 0;
}