Green公式-判断多边形边界曲线顺/逆时针

1double d = 0;
2for (int i = 0; i < n - 1; i++) {
3    d += -0.5 * ( y[i + 1] + y[i]) * (x[i + 1] - x[i]);
4}
5if ( d > 0)
6    cout << "counter clockwise" << endl;
7else
8    cout << "clockwise" << endl;