求多边形面积模板***

double area(){//top为总点数，即0~top-1，res为点数组
    double sum;
    sum = res[0].y * (res[top-1].x-res[1].x);
    for(int i = 1; i < top; i ++)
        sum += p[res[i]].y * (res[i-1].x-res[(i+1)%top].x);
    return sum / 2;
}