HDU 2202 最大三角形

题解：先算出凸包，然后枚举凸包上的点计算即可

#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <iostream>
using namespace std;
const int N = 50005;
const double eps = 1e-8;
struct point {int x,y;}p[N], stack[N];
bool isZero(double x){return (x>0?x:-x)<eps;}
double dis(point A, point B){return sqrt((A.x-B.x)*(A.x-B.x)+(A.y-B.y)*(A.y-B.y));}
int crossProd(point A, point B, point C){return (B.x-A.x)*(C.y-A.y)-(B.y-A.y)*(C.x-A.x);}
int cmp(const void *a, const void *b){
    point*c=(point*)a;
    point*d=(point*)b;
    double k = crossProd(p[0], *c, *d);
    if (kdis(p[0], *d))) return 1;
    return -1;
}
int Graham(int n) {
    int x = p[0].x;
    int y = p[0].y;
    int mi = 0;
    for (int i=1; i<n; ++i) {
        if (p[i].x=2)--top;
        stack[++top]=p[i];
    }
    return top;    
}
double maxTrangle(int n) {
    int top=Graham(n);
    double area,maxArea = 0;
    for (int i=0;i<top;++i) {
        for (int j=i+1;j<top;++j) {
            for (int k=j+1;k<=top;++k) {
                area=crossProd(stack[i],stack[j],stack[k]);
                if (maxArea<area) maxArea=area; 
            }
        }
    }
    return maxArea*0.5;
}
int main() {
    int n;
    while (scanf("%d", &n) != EOF) {
        for (int i=0; i<n; ++i) scanf ("%d%d", &p[i].x, &p[i].y);
        double ans = maxTrangle(n);
        printf ("%.2lf\n", ans);
    }
    return 0;
}