Codeforces Round #198 (Div. 二) B. Maximal Area Quadrilateral

Codeforces Round #198 (Div. 2) B. Maximal Area Quadrilateral
B. Maximal Area Quadrilateraltime limit per test1 secondmemory limit per test256 megabytesinputstandard inputoutputstandard output

Iahub has drawn a set of n points in the cartesian plane which he calls "special points". A quadrilateral is a simple polygon without self-intersections with four sides (also called edges) and four vertices (also called corners). Please note that a quadrilateral doesn't have to be convex. A special quadrilateral is one which has all four vertices in the set of special points. Given the set of special points, please calculate the maximal area of a special quadrilateral.

The first line contains integer n (4?≤?n?≤?300). Each of the next n lines contains two integers: xi, yi (?-?1000?≤?xi,?yi?≤?1000) — the cartesian coordinates of ith special point. It is guaranteed that no three points are on the same line. It is guaranteed that no two points coincide.

Output a single real number — the maximal area of a special quadrilateral. The answer will be considered correct if its absolute or relative error does't exceed 10?-?9.

50 00 44 04 42 3

16.000000

In the test example we can choose first 4 points to be the vertices of the quadrilateral. They form a square by side 4, so the area is 4·4?=?16.

题 意就是找4个点,求最大的四边形的面积,枚举对角线,我们可以求出上三角形的最大值,和下三角形的最大值,这样,我们就可以得出最大的四边形的面积,用叉积可以得出面积,还可以通过其正负,得出是上三角形,还是下三角形,这样,就可以得到了n^3的算法了!

#include <iostream>#include <stdio.h>#include <string.h>using namespace std;#define M 350#define eps 0#define inf 10000000000struct node {    double x,y;}p[M];double mul(int i,int j,int k){    return ((p[k].x-p[i].x)*(p[k].y-p[j].y)-(p[k].y-p[i].y)*(p[k].x-p[j].x))/2.0;}int main(){    int n,i,j,k;    while(scanf("%d",&n)!=EOF){        for(i=0;i<n;i++){            scanf("%lf%lf",&p[i].x,&p[i].y);        }        double lmax=-inf,rmax=-inf,amax=-inf;        for(i=0;i<n;i++){            for(j=0;j<n;j++){                if(i==j)                continue;                lmax=-inf,rmax=-inf;                for(k=0;k<n;k++){                    if(i==k||j==k)                    continue;                    double temp=mul(i,j,k);                    if(temp<eps){                        lmax=max(lmax,-temp);                    }                    else {                        rmax=max(rmax,temp);                    }                }                amax=max(amax,lmax+rmax);               // printf("%.6f %.6ffdsf\n",amax,lmax+rmax);            }        }        printf("%.6f\n",amax);    }    return 0;}