【POJ】2253 Frogger 二分+bfs

#include <cmath>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std ;

#define REP( i , a , b ) for ( int i = ( a ) ; i <  ( b ) ; ++ i )
#define FOR( i , a , b ) for ( int i = ( a ) ; i <= ( b ) ; ++ i )
#define REV( i , a , b ) for ( int i = ( a ) ; i >= ( b ) ; -- i )
#define travel( e , H , u ) for ( Edge* e = H[u] ; e ; e = e -> next )
#define CLR( a , x ) memset ( a , x , sizeof a )

const int MAXN = 1005 ;
const int MAXE = 1000005 ;
const double eps = 1e-6 ;

struct Edge {
	int v ;
	Edge* next ;
} E[MAXE] , *H[MAXN] , *cur ;

struct Point {
	int x , y ;
} p[MAXN] ;

bool vis[MAXN] ;
int Q[MAXN] , head , tail ;
int n ;

void clear () {
	cur = E ;
	CLR ( H , 0 ) ;
}

void addedge ( int u , int v ) {
	cur -> v = v ;
	cur -> next = H[u] ;
	H[u] = cur ++ ;
}

double dist ( int i , int j ) {
	double x = p[i].x - p[j].x ;
	double y = p[i].y - p[j].y ;
	return sqrt ( x * x + y * y ) ;
}

bool bfs () {
	CLR ( vis , 0 ) ;
	head = tail = 0 ;
	Q[tail ++] = 0 ;
	vis[0] = 1 ;
	while ( head != tail ) {
		int u = Q[head ++] ;
		travel ( e , H , u ) {
			int v = e -> v ;
			if ( !vis[v] ) {
				vis[v] = 1 ;
				Q[tail ++] = v ;
			}
		}
	}
	return vis[1] ;
}

double solve () {
	REP ( i , 0 , n ) scanf ( "%d%d" , &p[i].x , &p[i].y ) ;
	double l = 0 , r = 2000 ;
	while ( r - l > eps ) {
		double m = ( l + r ) / 2 ;
		clear () ;
		REP ( i , 0 , n )
			REP ( j , i + 1 , n )
				if ( dist ( i , j ) <= m ) {
					addedge ( i , j ) ;
					addedge ( j , i ) ;
				}
		if ( bfs () ) r = m ;
		else l = m ;
	}
	return l ;
}

int main () {
	int cas = 0 ;
	while ( ~scanf ( "%d" , &n ) && n ) printf ( "Scenario #%d\nFrog Distance = %.3f\n\n" , ++ cas , solve () ) ;
	return 0 ;
}