UVA 11853 Paintball ——（dfs+圆交判定）

 1 #include <stdio.h>
 2 #include <algorithm>
 3 #include <string.h>
 4 #include <set>
 5 #include <math.h>
 6 using namespace std;
 7 const int N = 1000 + 5;
 8 const int inf = 2e9;
 9 typedef long long ll;
10 typedef pair<int,int> pii;
11 
12 int n;
13 int vis[N];
14 struct circle
15 {
16     int x,y,r;
17     void read()
18     {
19         scanf("%d%d%d",&x,&y,&r);
20     }
21 }c[N];
22 
23 bool can(circle a,circle b)
24 {
25     double dx = a.x - b.x;
26     double dy = a.y - b.y;
27     double d = sqrt(dx*dx+dy*dy);
28     return d <= a.r + b.r;
29 }
30 
31 double ansL = 0, ansR = 0;
32 void update(int u)
33 {
34     if(c[u].x <= c[u].r)
35     {
36         double t = sqrt(c[u].r*c[u].r - c[u].x*c[u].x);
37         double pos = c[u].y - t;
38         ansL = min(ansL, pos);
39     }
40     if(c[u].x + c[u].r >= 1000)
41     {
42         double tt = 1000.0 - c[u].x;
43         double t = sqrt(c[u].r*c[u].r - tt*tt);
44         double pos = c[u].y - t;
45         ansR = min(ansR, pos);
46     }
47 }
48 
49 bool dfs(int u)
50 {
51     vis[u] = 1;
52     if(c[u].y <= c[u].r) return false;
53     update(u);
54     for(int i=1;i<=n;i++)
55     {
56         if(!vis[i] && can(c[u], c[i]))
57         {
58             if(!dfs(i)) return false;
59         }
60     }
61     return true;
62 }
63 
64 void solve()
65 {
66     ansL = 1000, ansR = 1000;
67     memset(vis,0,sizeof(vis));
68     for(int i=1;i<=n;i++)
69     {
70         if(!vis[i] && c[i].y + c[i].r >= 1000)
71         {
72             if(!dfs(i))
73             {
74                 puts("IMPOSSIBLE");
75                 return ;
76             }
77         }
78     }
79     printf("0.00 %.2f 1000.00 %.2f\n",ansL,ansR);
80 }
81 
82 int main()
83 {
84     while(scanf("%d",&n) == 1)
85     {
86         for(int i=1;i<=n;i++) c[i].read();
87         solve();
88     }
89     return 0;
90 }