1 /************************************************************** 2 Problem: 1337 3 User: idy002 4 Language: C++ 5 Result: Accepted 6 Time:4 ms 7 Memory:2372 kb 8 ****************************************************************/ 9 10 #include <cstdio> 11 #include <cmath> 12 #include <algorithm> 13 #define line(a,b) ((b)-(a)) 14 #define eps 1e-10 15 #define N 100010 16 using namespace std; 17 18 int sg( double x ) { return (x>-eps)-(x<eps); } 19 struct Vector { 20 double x, y; 21 Vector( double x=0, double y=0 ):x(x),y(y){} 22 Vector operator+( const Vector &b ) const { return Vector(x+b.x,y+b.y); } 23 Vector operator-( const Vector &b ) const { return Vector(x-b.x,y-b.y); } 24 Vector operator*( double b ) const { return Vector(x*b,y*b); } 25 Vector operator/( double b ) const { return Vector(x/b,y/b); } 26 double operator^( const Vector &b ) const { return x*b.y-y*b.x; } 27 double len() { return sqrt(x*x+y*y); } 28 Vector nor() { return Vector(-y,x); } 29 }; 30 typedef Vector Point; 31 Point inter( Point p, Vector u, Point q, Vector v ) { 32 return p+u*((line(p,q)^v)/(u^v)); 33 } 34 struct Circle { 35 Point o; 36 double r; 37 Circle(){} 38 Circle( Point a ):o(a),r(0){} 39 Circle( Point a, Point b ) { 40 o = (a+b)/2; 41 r = (a-b).len()/2; 42 } 43 Circle( Point a, Point b, Point c ) { // ab^bc != 0 44 Point p=(a+b)/2, q=(b+c)/2; 45 Vector u=(a-b).nor(), v=(b-c).nor(); 46 o = inter(p,u,q,v); 47 r = (o-a).len(); 48 } 49 bool contain( Point a ) { 50 return sg( (a-o).len() - r ) <= 0; 51 } 52 }; 53 54 int n; 55 Point pts[N]; 56 57 int main() { 58 scanf( "%d", &n ); 59 for( int i=1; i<=n; i++ ) { 60 double x, y; 61 scanf( "%lf%lf", &x, &y ); 62 pts[i] = Point(x,y); 63 } 64 random_shuffle( pts+1, pts+1+n ); 65 Circle c = Circle(pts[1]); 66 for( int i=2; i<=n; i++ ) { 67 if( c.contain(pts[i]) ) continue; 68 c = Circle(pts[i]); 69 for( int j=1; j<i; j++ ) { 70 if( c.contain(pts[j]) ) continue; 71 c = Circle(pts[i],pts[j]); 72 for( int k=1; k<j; k++ ) { 73 if( c.contain(pts[k]) ) continue; 74 c = Circle(pts[i],pts[j],pts[k]); 75 } 76 } 77 } 78 printf( "%.3lf ", c.r ); 79 }
题解见bzoj 1336