最远点对问题 >> NoAlGo博客


最远点对问题 » NoAlGo博客
二维平面上有n个点,怎么寻找距离最远的两个?
凸包(Convex Hull)是一个计算几何的概念,点集Q的凸包是指一个最小凸多边形,满足Q中的点或者在多边形边上或者在其内。直观上来讲,想象n个顶点上各有一枚钉子,使用一根橡皮圈把这所有钉子包围起来得到的图像就是这n个点的凸包。
不难想象,距离最远的点肯定在凸包的顶点上,于是可以先对所有的点求一个凸包,然后再对凸包上的点求最短距离。一般来说,凸包上的点的数量会比原始点的数量少很多,这时我们可以对凸包上的所有进行暴力枚举,所花费的时间不会太大。但对于特殊构造的数据,可能所有的n个点就构成了一个凸包,这时在凸包上枚举跟直接在原始点集上枚举是一样的,复杂度仍然为O(n^2),这时我们可以选择使用旋转卡壳方法求取最远点对。
旋转卡壳算法
struct Point
{
 double x, y;
 Point (double a = 0, double b = 0): x(a), y(b) {}
} v[mxsz]; //顶点集
int stk[mxsz], top; //graham扫描得到的凸包顶点下标

//计算欧几里得距离
inline double calDis(const Point &p1, const Point &p2)
{
 return sqrt((p1.x-p2.x)*(p1.x-p2.x) + (p1.y-p2.y)*(p1.y-p2.y));
}

//计算向量ab和向量cd的叉乘
double cross(const Point &a, const Point &b, const Point &c, const Point &d)
{
 double x1 = b.x - a.x, y1 = b.y - a.y, x2 = d.x - c.x, y2 = d.y - c.y;
 return x1 * y2 - x2 * y1;
}

bool cmp(const Point &a, const Point &b)
{
 double t = cross(v[0], a, v[0], b);
 if (fabs(t) < 1e-6) return calDis(v[0], a) < calDis(v[0], b);
 else return t > 0; //右手法则,叉乘为正,则由a到b为逆时针
}

//graham扫描法求凸包
void graham (int n)
{
 //最左下角的点放到首位
 int id = 0; 
 for (int i = 1; i < n; i++)
  if (v[i].y < v[id].y || (v[i].y == v[id].y && v[i].x < v[id].x))
   id = i;
 swap(v[0], v[id]);

 //极角排序
 sort(v+1, v+n, cmp);

 //前两个点直接入栈
 for (int i = 0; i <= 1; i++) stk[i] = i;
 top = 1;

 for (int i = 2; i < n; i++)
 {
  while (top > 0 && cross(v[stk[top-1]], v[stk[top]], v[stk[top-1]], v[i]) <= 0)
   top--; //共线的点只保留两端,如果n个点均共线,则最终只得到首尾两个端点
  stk[++top] = i;
 }
}

//旋转卡壳求凸包的直径
double rorating_caliper()
{
 double ans = 0;
 stk[++top] = 0; //最后一个点即为最开始的点
 for (int i = 0, j = 2; i < top; i++)
 {
  while (cross(v[stk[i]], v[stk[i+1]], v[stk[i]], v[stk[j]]) 
    < cross(v[stk[i]], v[stk[i+1]], v[stk[i]], v[stk[j+1]]))
    j = (j + 1) % top;
  ans = max(ans, max(calDis(v[stk[i]], v[stk[j]]), calDis(v[stk[i+1]], v[stk[j]])));
 }
 return ans;
}

int main()
{
 //输入n个点
 int n; scanf("%d", &n);
 for (int i = 0; i < n; i++) scanf("%lf%lf", &v[i].x, &v[i].y);

 //graham扫描法求凸包
 graham(n);

 //旋转卡壳求最远距离
 double len = rorating_caliper();
 printf("Longest distance is: %lf\n", len);
}
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