点对多边形的夹角
struct point{double x, y;};double get_angle(point p, point ps[], int n){int i;//过p做一条水平直线，求所有点对这条水平直线的夹角，最大的角减去最小的角即可double Max, Min, ang1, ang2, dang, sum;Max = Min = sum = 0;ps[n] = ps[0];//ang2 = atan2(ps[0].y - p.y, ps[0].x - p.x);for(i = 1; i <= n; i++){//ang1 = ang2;ang1 = atan2(ps[i-1].y - p.y, ps[i-1].x - p.x);ang2 = atan2(ps[i].y - p.y, ps[i].x - p.x);if(ang1 - ang2 > pi) ang2 += 2 * pi;if(ang2 - ang1 > pi) ang1 += 2 * pi;sum += ang1 - ang2;if(sum > Max) Max = sum;if(sum < Min) Min = sum;if(Max - Min > 2 * pi){Max = Min + 2 * pi;break;}}return Max - Min;}