凸包(ConvexHull)就是把给定点包围在内部的，面积最小的凸多边形。时间复杂度为O(n)先计算凸包，输入点数组p，点个数为n，输出点数组ch。函数返回凸包顶点数。(The convex hull (ConvexHull) is the smallest convex polygon that surrounds the point to its interior. The time complexity is O (n). Fir