题目:http://www.lydsy.com/JudgeOnline/problem.php?id=1038
裸裸的半平面交,WA了N次之后实在感叹O2下的精度实在DT,改成long double之后还发现居然还可以输出-0的额。。。唉~
代码:
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>
using namespace std ;
#define rep( i , x ) for ( int i = 0 ; i ++ < x ; )
#define maxn 1010
typedef long double ld ;
const ld esp = 0.0000001 ;
const ld inf = ld( 0x7fffffff ) * ld( 0x7fffffff ) ;
struct point {
ld x , y ;
void oper( ld _x , ld _y ) {
x = _x , y = _y ;
}
} p[ maxn ] , pos[ maxn ] ;
struct line {
ld k , b ;
void oper( point x , point y ) {
k = ( x.y - y.y ) / ( x.x - y.x ) ;
b = x.y - k * x.x ;
}
point cross( line l ) {
point rec ;
rec.x = - ( b - l.b ) / ( k - l.k ) ;
rec.y = cal( rec.x ) ;
return rec ;
}
ld cal( ld x ) {
return k * x + b ;
}
bool operator < ( const line &a ) const {
return k < a.k || ( k == a.k && b > a.b ) ;
}
} li[ maxn ] , st[ maxn ] ;
int n , m = 0 , Top = 0 , M = 0 ;
ld calh( ld x , int left , int right , point *ret ) {
if ( abs( ret[ left ].x - x ) < esp ) return ret[ left ].y ;
if ( abs( ret[ right ].x - x ) < esp ) return ret[ right ].y ;
while ( right - left > 1 ) {
int mid = ( left + right ) >> 1 ;
if ( abs( ret[ mid ].x - x ) < esp ) return ret[ mid ].y ;
if ( x < ret[ mid ].x ) right = mid ; else left = mid ;
}
line temp ;
temp.oper( ret[ left ] , ret[ right ] ) ;
return temp.cal( x ) ;
}
ld calheight( ld x ) {
ld h1 = calh( x , 1 , M , pos ) , h2 = calh( x , 1 , n , p ) ;
return h1 - h2 ;
}
int main( ) {
scanf( "%d" , &n ) ;
rep( i , n ) {
double x ; scanf( "%lf" , &x ) ;
p[ i ].x = ld( x ) ;
}
rep( i , n ) {
double y ; scanf( "%lf" , &y ) ;
p[ i ].y = ld( y ) ;
}
rep( i , n - 1 ) li[ ++ m ].oper( p[ i ] , p[ i + 1 ] ) ;
sort( li + 1 , li + m + 1 ) ;
rep( i , m ) {
if ( abs( li[ i ].k - li[ i - 1 ].k ) <= esp ) continue ;
while ( Top > 1 ) {
point temp = st[ Top ].cross( li[ i ] ) ;
if ( st[ Top - 1 ].cal( temp.x ) >= temp.y ) -- Top ;
else break ;
}
st[ ++ Top ] = li[ i ] ;
}
rep( i , Top ) {
point temp = st[ i ].cross( st[ i + 1 ] ) ;
}
int left = 1 , right = Top ;
while ( left < right ) {
point temp = st[ left ].cross( st[ left + 1 ] ) ;
if ( temp.x <= p[ 1 ].x ) ++ left ; else break ;
}
while ( left < right ) {
point temp = st[ right ].cross( st[ right - 1 ] ) ;
if ( temp.x >= p[ n ].x ) -- right ; else break ;
}
pos[ ++ M ].oper( p[ 1 ].x , st[ left ].cal( p[ 1 ].x ) ) ;
for ( int i = left ; i < right ; ++ i ) {
pos[ ++ M ] = st[ i ].cross( st[ i + 1 ] ) ;
}
pos[ ++ M ].oper( p[ n ].x , st[ right ].cal( p[ n ].x ) ) ;
ld ans = inf ;
rep( i , M ) ans = min( ans , calheight( pos[ i ].x ) ) ;
rep( i , n ) ans = min( ans , calheight( p[ i ].x ) ) ;
printf( "%.3f\n" , double( ans + esp ) ) ;
return 0 ;
}