[BZOJ 3007] 拯救小云公主 对偶图

 1 #include <cstdio>
 2 #include <cstring>
 3 #include <cstdlib>
 4 #include <cctype>
 5 #include <cmath>
 6 #define F(i, a, b) for (register int i = (a); i <= (b); i++)
 7 #define db double
 8 inline int rd(void) {
 9     int f = 1; char c = getchar(); for (; !isdigit(c); c = getchar()) if (c == ‘-‘) f = -1;
10     int x = 0; for (; isdigit(c); c = getchar()) x = x*10+c-‘0‘; return x*f;
11 }
12 
13 const int N = 3005;
14 
15 int n, r, c, x[N], y[N];
16 int S, T, tot;
17 int f[N];
18 
19 inline int Find(int x) { return f[x] == x ? x : f[x] = Find(f[x]); }
20 inline void Union(int x, int y) { f[Find(x)] = Find(y); }
21 bool Check(db Min) {
22     F(i, 1, tot) f[i] = i;
23     F(i, 1, n) {
24         if (x[i] - Min < 1 || y[i] + Min > c) Union(S, i);
25         if (x[i] + Min > r || y[i] - Min < 1) Union(i, T);
26     }
27     F(i, 1, n) F(j, i+1, n)
28         if (1.0 * (x[i] - x[j]) * (x[i] - x[j]) + 1.0 * (y[i] - y[j]) * (y[i] - y[j]) <= 4 * Min * Min)
29             Union(i, j);
30     return Find(S) != Find(T);
31 }
32 
33 int main(void) {
34     #ifndef ONLINE_JUDGE
35         freopen("bzoj3007.in", "r", stdin);
36     #endif
37     
38     n = rd(), r = rd(), c = rd();
39     F(i, 1, n) x[i] = rd(), y[i] = rd();
40     tot = n, S = ++tot, T = ++tot;
41     
42     db L = 0, R = sqrt(1.0 * (r-1) * (r-1) + 1.0 * (c-1) * (c-1));
43     while (R-L > 1e-4) {
44         db M = (L+R)/2;
45         Check(M) ? L = M : R = M;
46     }
47     printf("%0.2lf\n", L);
48     
49     return 0;
50 }