roberthood.cpp

#include <bits/stdc++.h>
using namespace std;

#define EPS 1e-9
#define PI acos(-1.0)

float DEG_to_RAD(float d) { return d * PI / 180.0; }

float RAD_to_DEG(float r) { return r * 180.0 / PI; }

struct point { float x, y;   // only used if more precision is needed
  point() { x = y = 0.0; }                      // default constructor
  point(float _x, float _y) : x(_x), y(_y) {}        // user-defined
  bool operator == (point other) const {
   return (fabs(x - other.x) < EPS && (fabs(y - other.y) < EPS)); } };

struct vec { float x, y;  // name: `vec' is different from STL vector
  vec(float _x, float _y) : x(_x), y(_y) {} };

vec toVec(point a, point b) {       // convert 2 points to vector a->b
  return vec(b.x - a.x, b.y - a.y); }

float dist(point p1, point p2) {                // Euclidean distance
  return hypot(p1.x - p2.x, p1.y - p2.y); }           // return float

// returns the perimeter, which is the sum of Euclidian distances
// of consecutive line segments (polygon edges)
float perimeter(const vector<point> &P) {
  float result = 0.0;
  for (int i = 0; i < (int)P.size()-1; i++)  // remember that P[0] = P[n-1]
    result += dist(P[i], P[i+1]);
  return result; }

// returns the area, which is half the determinant
float area(const vector<point> &P) {
  float result = 0.0, x1, y1, x2, y2;
  for (int i = 0; i < (int)P.size()-1; i++) {
    x1 = P[i].x; x2 = P[i+1].x;
    y1 = P[i].y; y2 = P[i+1].y;
    result += (x1 * y2 - x2 * y1);
  }
  return fabs(result) / 2.0; }

float dot(vec a, vec b) { return (a.x * b.x + a.y * b.y); }

float norm_sq(vec v) { return v.x * v.x + v.y * v.y; }

float angle(point a, point o, point b) {  // returns angle aob in rad
  vec oa = toVec(o, a), ob = toVec(o, b);
  return acos(dot(oa, ob) / sqrt(norm_sq(oa) * norm_sq(ob))); }

float cross(vec a, vec b) { return a.x * b.y - a.y * b.x; }

// note: to accept collinear points, we have to change the `> 0'
// returns true if point r is on the left side of line pq
bool ccw(point p, point q, point r) {
  return cross(toVec(p, q), toVec(p, r)) > 0; }

// returns true if point r is on the same line as the line pq
bool collinear(point p, point q, point r) {
  return fabs(cross(toVec(p, q), toVec(p, r))) < EPS; }

// returns true if we always make the same turn while examining
// all the edges of the polygon one by one
bool isConvex(const vector<point> &P) {
  int sz = (int)P.size();
  if (sz <= 3) return false;   // a point/sz=2 or a line/sz=3 is not convex
  bool isLeft = ccw(P[0], P[1], P[2]);               // remember one result
  for (int i = 1; i < sz-1; i++)            // then compare with the others
    if (ccw(P[i], P[i+1], P[(i+2) == sz ? 1 : i+2]) != isLeft)
      return false;            // different sign -> this polygon is concave
  return true; }                                  // this polygon is convex

// returns true if point p is in either convex/concave polygon P
bool inPolygon(point pt, const vector<point> &P) {
  if ((int)P.size() == 0) return false;
  float sum = 0;    // assume the first vertex is equal to the last vertex
  for (int i = 0; i < (int)P.size()-1; i++) {
    if (ccw(pt, P[i], P[i+1]))
         sum += angle(P[i], pt, P[i+1]);                   // left turn/ccw
    else sum -= angle(P[i], pt, P[i+1]); }                 // right turn/cw
  return fabs(fabs(sum) - 2*PI) < EPS; }

// line segment p-q intersect with line A-B.
point lineIntersectSeg(point p, point q, point A, point B) {
  float a = B.y - A.y;
  float b = A.x - B.x;
  float c = B.x * A.y - A.x * B.y;
  float u = fabs(a * p.x + b * p.y + c);
  float v = fabs(a * q.x + b * q.y + c);
  return point((p.x * v + q.x * u) / (u+v), (p.y * v + q.y * u) / (u+v)); }

// cuts polygon Q along the line formed by point a -> point b
// (note: the last point must be the same as the first point)
vector<point> cutPolygon(point a, point b, const vector<point> &Q) {
  vector<point> P;
  for (int i = 0; i < (int)Q.size(); i++) {
    float left1 = cross(toVec(a, b), toVec(a, Q[i])), left2 = 0;
    if (i != (int)Q.size()-1) left2 = cross(toVec(a, b), toVec(a, Q[i+1]));
    if (left1 > -EPS) P.push_back(Q[i]);       // Q[i] is on the left of ab
    if (left1 * left2 < -EPS)        // edge (Q[i], Q[i+1]) crosses line ab
      P.push_back(lineIntersectSeg(Q[i], Q[i+1], a, b));
  }
  if (!P.empty() && !(P.back() == P.front()))
    P.push_back(P.front());        // make P's first point = P's last point
  return P; }

point pivot;
bool angleCmp(point a, point b) {                 // angle-sorting function
  if (collinear(pivot, a, b))                               // special case
    return dist(pivot, a) < dist(pivot, b);    // check which one is closer
  float d1x = a.x - pivot.x, d1y = a.y - pivot.y;
  float d2x = b.x - pivot.x, d2y = b.y - pivot.y;
  return (atan2(d1y, d1x) - atan2(d2y, d2x)) < 0; }   // compare two angles

vector<point> CH(vector<point> P) {   // the content of P may be reshuffled
  int i, j, n = (int)P.size();
  if (n <= 3) {
    if (!(P[0] == P[n-1])) P.push_back(P[0]); // safeguard from corner case
    return P;                           // special case, the CH is P itself
  }

  // first, find P0 = point with lowest Y and if tie: rightmost X
  int P0 = 0;
  for (i = 1; i < n; i++)
    if (P[i].y < P[P0].y || (P[i].y == P[P0].y && P[i].x > P[P0].x))
      P0 = i;

  point temp = P[0]; P[0] = P[P0]; P[P0] = temp;    // swap P[P0] with P[0]

  // second, sort points by angle w.r.t. pivot P0
  pivot = P[0];                    // use this global variable as reference
  sort(++P.begin(), P.end(), angleCmp);              // we do not sort P[0]

  // third, the ccw tests
  vector<point> S;
  S.push_back(P[n-1]); S.push_back(P[0]); S.push_back(P[1]);   // initial S
  i = 2;                                         // then, we check the rest
  while (i < n) {           // note: N must be >= 3 for this method to work
    j = (int)S.size()-1;
    if (ccw(S[j-1], S[j], P[i])) S.push_back(P[i++]);  // left turn, accept
    else S.pop_back(); }   // or pop the top of S until we have a left turn
  return S; }   

int main() {
  int n; cin >> n;
  vector<point> p;
  for(int i = 0; i < n; i++) {
    int x, y; cin >> x >> y;
    p.push_back(point(x, y));
  }
  float maxDist = 0;
  if(p.size() > 20) {
    vector<point> ch = CH(p);
    for(int i = 0; i < ch.size(); i++) {
      for(int j = i+1; j < ch.size(); j++) {
        maxDist = max(maxDist, dist(ch[i], ch[j]));
      }
    }
  } else {
    for(int i = 0; i < p.size(); i++) {
      for(int j = i+1; j < p.size(); j++) {
        maxDist = max(maxDist, dist(p[i], p[j]));
      }
    }
  }
  printf("%.8f\n", maxDist);
}