Problem p02301 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
typedef complex<double> P;
typedef vector<P> vec;

namespace std {
bool operator<(const P &a, const P &b) {
  return (a.imag() == b.imag() ? a.real() < b.real() : a.imag() < b.imag());
}
}; // namespace std

double eps = 1e-8;
double PI = acos(-1);

bool eq(double a, double b) { return (b - a < eps && a - b < eps); }
bool eq(P a, P b) { return (abs(a - b) < eps); }

double dot(P a, P b) { return real(b * conj(a)); }
double cross(P a, P b) { return imag(b * conj(a)); }
P project(P a, P b, P c) {
  b -= a;
  c -= a;
  return a + b * real(c / b);
}
P reflect(P a, P b, P c) {
  b -= a;
  c -= a;
  return a + conj(c / b) * b;
}
int ccw(P a, P b, P c) {
  b -= a, c -= a, a = c * conj(b);
  if (a.imag() > eps)
    return 1; // ccw
  if (a.imag() < -eps)
    return -1; // cw
  if (a.real() < -eps)
    return 2; // ob
  if (abs(b) + eps < abs(c))
    return -2; // of
  return 0;    // os
}

// segment ab , point c
double dist(P a, P b, P c) {
  if (dot(b - a, c - a) < 0)
    return abs(c - a);
  if (dot(a - b, c - b) < 0)
    return abs(c - b);
  return abs(cross(b - a, c - a)) / abs(b - a);
}

bool isintersect(P a, P b, P c, P d) {
  return ((ccw(a, b, c) * ccw(a, b, d) <= 0) &&
          (ccw(c, d, a) * ccw(c, d, b) <= 0));
}

// segment ab , segment cd
P intersect(P a, P b, P c, P d) {
  a -= d;
  b -= d;
  c -= d;
  return d + a + (b - a) * imag(a / c) / imag(a / c - b / c);
}

double dist(P a, P b, P c, P d) {
  if (isintersect(a, b, c, d))
    return 0;
  double ab = min(dist(a, b, c), dist(a, b, d));
  double cd = min(dist(c, d, a), dist(c, d, b));
  return min(ab, cd);
}

double calcArea(vec &t) {
  double res = 0;
  int n = t.size();
  for (int i = 0; i < n; i++)
    res += cross(t[i], t[(i + 1 == n ? 0 : i + 1)]);
  return abs(res / 2.0);
}

double Arg(P a, P b, P c) {
  b -= a, c -= a;
  return arg(c * conj(b));
}

int inPolygon(vec &t, P p) {
  int n = t.size();
  double sum = 0;
  for (int i = 0; i < n; i++) {
    P a = t[i], b = t[(i + 1 == n ? 0 : i + 1)];
    if (ccw(a, b, p) == 0)
      return 1; // on
    sum += Arg(p, a, b);
  }
  if (abs(sum) < eps)
    return 0; // out
  else
    return 2; // in
}

typedef pair<double, P> Pair;

vec andrewScan(vec &t) {
  int N = t.size(), C = 0;
  vec R(N);
  for (int i = 0; i < N; i++) {
    while (2 <= C && ccw(R[C - 2], R[C - 1], t[i]) == -1)
      C--;
    R[C++] = t[i];
  }
  vec res(C);
  for (int i = 0; i < C; i++)
    res[i] = R[i];
  return res;
}

vec ConvexHull2(vec t) {
  sort(t.begin(), t.end());
  vec u = andrewScan(t);
  reverse(t.begin(), t.end());
  vec l = andrewScan(t);
  for (int i = 1; i + 1 < (int)l.size(); i++)
    u.push_back(l[i]);
  return u;
}

vec ConvexHull(vec t) {
  vector<Pair> u;
  vec R;
  int N = t.size(), K = 0, C = 0;
  for (int i = 1; i < N; i++)
    if (t[i] < t[K])
      K = i;
  for (int i = 0; i < N; i++)
    u.push_back(Pair(arg(t[i] - t[K]), t[i]));
  R.push_back(t[K]), C++;
  sort(u.begin(), u.end());
  for (int i = 0; i < N; i++) {
    P p = u[i].second;
    if (2 <= C && ccw(R[C - 2], R[C - 1], p) == 0)
      continue;
    if (2 <= C && ccw(R[C - 2], R[C - 1], p) == 2)
      continue;
    while (2 <= C && ccw(R[C - 2], R[C - 1], p) != 1)
      R.pop_back(), C--;
    R.push_back(p), C++;
  }
  return R;
}

int main() {
  int n;
  cin >> n;
  vec t;
  double x, y;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    t.push_back(P(x, y));
  }
  vec ans = ConvexHull2(t);
  int m = ans.size();
  double maxm = 0;

  for (int i = 0; i < m; i++) {
    for (int j = 0; j < m; j++) {
      maxm = max(maxm, abs(ans[i] - ans[j]));
    }
  }

  printf("%.8f\n", maxm);
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef complex<double> P;
typedef vector<P> vec;

namespace std {
bool operator<(const P &a, const P &b) {
  return (a.imag() == b.imag() ? a.real() < b.real() : a.imag() < b.imag());
}
}; // namespace std

double eps = 1e-8;
double PI = acos(-1);

bool eq(double a, double b) { return (b - a < eps && a - b < eps); }
bool eq(P a, P b) { return (abs(a - b) < eps); }

double dot(P a, P b) { return real(b * conj(a)); }
double cross(P a, P b) { return imag(b * conj(a)); }
P project(P a, P b, P c) {
  b -= a;
  c -= a;
  return a + b * real(c / b);
}
P reflect(P a, P b, P c) {
  b -= a;
  c -= a;
  return a + conj(c / b) * b;
}
int ccw(P a, P b, P c) {
  b -= a, c -= a, a = c * conj(b);
  if (a.imag() > eps)
    return 1; // ccw
  if (a.imag() < -eps)
    return -1; // cw
  if (a.real() < -eps)
    return 2; // ob
  if (abs(b) + eps < abs(c))
    return -2; // of
  return 0;    // os
}

// segment ab , point c
double dist(P a, P b, P c) {
  if (dot(b - a, c - a) < 0)
    return abs(c - a);
  if (dot(a - b, c - b) < 0)
    return abs(c - b);
  return abs(cross(b - a, c - a)) / abs(b - a);
}

bool isintersect(P a, P b, P c, P d) {
  return ((ccw(a, b, c) * ccw(a, b, d) <= 0) &&
          (ccw(c, d, a) * ccw(c, d, b) <= 0));
}

// segment ab , segment cd
P intersect(P a, P b, P c, P d) {
  a -= d;
  b -= d;
  c -= d;
  return d + a + (b - a) * imag(a / c) / imag(a / c - b / c);
}

double dist(P a, P b, P c, P d) {
  if (isintersect(a, b, c, d))
    return 0;
  double ab = min(dist(a, b, c), dist(a, b, d));
  double cd = min(dist(c, d, a), dist(c, d, b));
  return min(ab, cd);
}

double calcArea(vec &t) {
  double res = 0;
  int n = t.size();
  for (int i = 0; i < n; i++)
    res += cross(t[i], t[(i + 1 == n ? 0 : i + 1)]);
  return abs(res / 2.0);
}

double Arg(P a, P b, P c) {
  b -= a, c -= a;
  return arg(c * conj(b));
}

int inPolygon(vec &t, P p) {
  int n = t.size();
  double sum = 0;
  for (int i = 0; i < n; i++) {
    P a = t[i], b = t[(i + 1 == n ? 0 : i + 1)];
    if (ccw(a, b, p) == 0)
      return 1; // on
    sum += Arg(p, a, b);
  }
  if (abs(sum) < eps)
    return 0; // out
  else
    return 2; // in
}

typedef pair<double, P> Pair;

vec andrewScan(vec &t) {
  int N = t.size(), C = 0;
  vec R(N);
  for (int i = 0; i < N; i++) {
    while (2 <= C && ccw(R[C - 2], R[C - 1], t[i]) == -1)
      C--;
    R[C++] = t[i];
  }
  vec res(C);
  for (int i = 0; i < C; i++)
    res[i] = R[i];
  return res;
}

vec ConvexHull2(vec t) {
  sort(t.begin(), t.end());
  vec u = andrewScan(t);
  reverse(t.begin(), t.end());
  vec l = andrewScan(t);
  for (int i = 1; i + 1 < (int)l.size(); i++)
    u.push_back(l[i]);
  return u;
}

vec ConvexHull(vec t) {
  vector<Pair> u;
  vec R;
  int N = t.size(), K = 0, C = 0;
  for (int i = 1; i < N; i++)
    if (t[i] < t[K])
      K = i;
  for (int i = 0; i < N; i++)
    u.push_back(Pair(arg(t[i] - t[K]), t[i]));
  R.push_back(t[K]), C++;
  sort(u.begin(), u.end());
  for (int i = 0; i < N; i++) {
    P p = u[i].second;
    if (2 <= C && ccw(R[C - 2], R[C - 1], p) == 0)
      continue;
    if (2 <= C && ccw(R[C - 2], R[C - 1], p) == 2)
      continue;
    while (2 <= C && ccw(R[C - 2], R[C - 1], p) != 1)
      R.pop_back(), C--;
    R.push_back(p), C++;
  }
  return R;
}

int main() {
  int n;
  cin >> n;
  vec t;
  double x, y;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    t.push_back(P(x, y));
  }
  vec ans = ConvexHull(t);
  int m = ans.size();
  double maxm = 0;

  for (int i = 0; i < m; i++) {
    for (int j = 0; j < m; j++) {
      maxm = max(maxm, abs(ans[i] - ans[j]));
    }
  }

  printf("%.8f\n", maxm);
  return 0;
}