Uva 10382 (区间覆盖) Watering Grass

 1 //#define LOCAL
 2 #include <vector>
 3 #include <cstdio>
 4 #include <cmath>
 5 #include <algorithm>
 6 #include <functional>
 7 using namespace std;
 8 
 9 const int MAXN = 10240;
10 const double EPS = 1e-11;
11 
12 struct Range
13 {
14     double a, b;
15     inline bool operator< (const Range& rhs) const
16     {
17         return a < rhs.a || (a == rhs.a && b < rhs.b);
18     }
19 };
20 
21 int main(void)
22 {
23     #ifdef LOCAL
24         freopen("10382in.txt", "r", stdin);
25     #endif
26 
27     int n, l, w;
28     double lenth, width;
29     vector<Range> ranges;
30     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
31 
32     while(scanf("%d%d%d", &n, &l, &w) == 3)
33     {
34         lenth = (double)l;
35         width = w / 2.0;
36         ranges.clear();
37 
38         for(int i = 0; i < n; ++i)
39         {
40             int position, radius;
41             double xw;
42             Range range;
43 
44             scanf("%d%d", &position, &radius);
45             if(radius * 2 <= w)    continue;
46             xw = sqrt((double)radius * radius - width * width);
47 
48             range.a = position - xw;
49             if(range.a > lenth + EPS)    continue;
50             else if(range.a - EPS < 0.0)    range.a = 0.0;
51             range.b = position + xw;
52             ranges.push_back(range);
53         }
54 
55         sort(ranges.begin(), ranges.end());
56 
57         int minCover = 0;
58         double start = 0.0, end = 0.0;
59         for(vector<Range>::iterator pr = ranges.begin(); pr != ranges.end(); )
60         {
61             start = end;
62             if(pr->a > start + EPS)
63             {
64                 minCover = -1;
65                 break;
66             }
67             ++minCover;
68             while(pr != ranges.end() && pr->a <= start)
69             {
70                 if(pr->b > end + EPS)    end = pr->b;
71                 ++pr;
72             }
73             if(end > lenth + EPS)
74                 break;
75         }
76         if(end + EPS < lenth)    minCover = -1;
77         printf("%d\n", minCover);
78     }
79     return 0;
80 }