Programación Competitiva

/**
 * Author: Ulf Lundstrom
 * Date: 2009-03-21
 * License: CC0
 * Source: tinyKACTL
 * Description: Returns twice the signed area of a polygon.
 *  Clockwise enumeration gives negative area. Watch out for overflow if using int as T!
 * Status: Tested with unitTest, Kattis problems polygonarea and wrapping and UVa Online Judge Problem: 109 - SCUD Busters
 */
#pragma once

#include "Point.h"

template <class T>
T polygonArea2(vector<Point<T>>& v) {
	T a = v.back().cross(v[0]);
	FOR(i,0,sz(v)-1) a += v[i].cross(v[i+1]);
	return a;
}