POJ 3348 Cows 求凸包面积

题目描述：http://poj.org/problem?id=3348

解题思路：先求凸包然后求面积。

/*
 *  2014.11.28  
 *  Problem: 3348		
 *  Memory: 180K		Time: 16MS
 *  Language: C++		Result: Accepted
 *
 */ 
#include "iostream"
#include "cmath"
#include "algorithm"
#define MAXN 10007
#define EPS 1e-8
using namespace std;

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

int nt, n = 0;

double cross(Point a, Point b, Point c) {
	double x1 = b.x-a.x, y1 = b.y-a.y;
	double x2 = c.x-a.x, y2 = c.y-a.y;
	return (x1*y2-x2*y1);
}

bool turnLeft(Point a, Point b, Point c) {
	return cross(a, b, c)>=-EPS;
}

bool cmp(Point a, Point b) {
	return a.x<b.x || (a.x==b.x && a.y<b.y);
}

void convexHull(int &n, Point p[], int &nt, Point t[]) {
	sort(t+1, t+nt+1, cmp);
	p[n=1] = t[1];
	for (int i=2; i<=nt; i++) {
		while (n>=2 && turnLeft(p[n-1], p[n], t[i])) n--;
		p[++n] = t[i];
	}
	int pn = n;
	for (int i=nt-1;i>=1;i--) {
		while (n>=pn+1 && turnLeft(p[n-1], p[n], t[i])) n--;
		p[++n] = t[i];
	}	
	n--;
}

double getArea(Point p[]) {
	double ans = 0;
	for (int i=2;i<n;i++) {
		ans += fabs(cross(p[1], p[i], p[i+1]));
	}
	return ans*0.5;
}

int main() {
	scanf("%d", &nt);
	for (int i=1;i<=nt;i++) scanf("%lf %lf", &t[i].x, &t[i].y);
	convexHull(n, p, nt, t);
	double area = getArea(p);
	printf("%d\n", (int)area/50);
	return 0;
}