[codevs 1249]多边形的面积

题解：

模板，利用Cross叉积表示的是有向面积，同样可以求凹多边形的面积。

代码：

总时间耗费: 5ms 总内存耗费: 256 kB

;const int maxn = 100 + 10;int n;struct Point {int x, y;Point(int x=0, int y=0):x(x),y(y) {}}P[maxn];typedef Point Vector;Vector operator + (Vector A, Vector B) { return Vector(A.x+B.x, A.y+B.y); }Vector operator – (Vector A, Vector B) { return Vector(A.x-B.x, A.y-B.y); }Vector operator * (Vector A, int p) { return Vector(A.x*p, A.y*p); }Vector operator / (Vector A, int p) { return Vector(A.x/p, A.y/p); }bool operator < (const Vector& a, const Vector& b) {return a.x < b.x || (a.x == b.x && a.y < b.y);}int Dot(Vector A, Vector B) { return A.x*B.x + A.y*B.y; }int Length(Vector A) { return sqrt(Dot(A, A)); }int Angle(Vector A, Vector B) { return acos(Dot(A, B) / Length(A) / Length(B)); }int Cross(Vector A, Vector B) { return A.x*B.y – A.y*B.x; }int Area2(Point A, Point B, Point C) { return Cross(B-A, C-A); }int PolygonArea() {int area = 0;for(int i = 1; i < n-1; i++)area += Cross(P[i]-P[0], P[i+1]-P[0]);return area/2;}int main() {scanf(“%d”, &n);for(int i = 0; i < n; i++)scanf(“%d%d”, &P[i].x, &P[i].y);printf(“%d\n”, PolygonArea());return 0;}

，十七岁怎么会有七十岁的忧伤，十八岁怎么会有八十岁的等待。