Problem p00508 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <ctime>
#include <iostream>
#include <vector>

using namespace std;

#define eps 0.0000000001

struct vec {
  double x, y;
  void nextvec() {
    scanf("%lf %lf", &x, &y);
    return;
  }
  vec operator+(const vec &a) const { return (vec){x + a.x, y + a.y}; }
  vec operator-(const vec &a) const { return (vec){x - a.x, y - a.y}; }
  vec operator*(const double &k) const { return (vec){k * x, k * y}; }
  vec sca(double t) { return (vec){t * x, t * y}; }
  double dot(vec a) { return x * a.x + y * a.y; }
  double cross(vec a) { return x * a.y - y * a.x; }
  double norm() { return sqrt(x * x + y * y); }
  double norm2() { return x * x + y * y; }
};

bool xsort(vec qwe, vec rty) { return qwe.x < rty.x; }
bool ysort(vec qwe, vec rty) { return qwe.y < rty.y; }

vec pt[100000];
vec ytmp[100000];
vec ymerge[100000];
double ma = 100000000000.0;

double closestpair(int l, int r) {
  if (r - l <= 1)
    return ma;
  int m = l + (r - l) / 2;
  double xm = pt[m].x;
  double res = min(closestpair(l, m), closestpair(m, r));
  double d = sqrt(res);

  int hdmerge = 0, hdtmp = 0, lhd = l, rhd = m;
  vec now;
  for (;;) {
    if (lhd == m && rhd == r)
      break;
    else if (rhd == r)
      now = pt[lhd++];
    else if (lhd == m)
      now = pt[rhd++];
    else if (ysort(pt[lhd], pt[rhd]))
      now = pt[lhd++];
    else
      now = pt[rhd++];

    ymerge[hdmerge++] = now;
    if (abs(now.x - xm) <= res + eps) {
      ytmp[hdtmp++] = now;
      for (int i = hdtmp - 2; i >= 0; --i) {
        if (abs(ytmp[i].y - now.y) >= d - eps)
          break;
        res = min(res, (now - ytmp[i]).norm2());
        d = sqrt(res);
      }
    }
  }
  for (int i = l; i < r; ++i)
    pt[i] = ymerge[i - l];

  return (int)res;
}

signed main(void) {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; ++i)
    pt[i].nextvec();
  sort(pt, pt + n, xsort);
  printf("%d\n", (int)closestpair(0, n));

  return 0;
}

Fixed submission:
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <ctime>
#include <iostream>
#include <vector>

using namespace std;

#define eps 0.0000000001

struct vec {
  double x, y;
  void nextvec() {
    scanf("%lf %lf", &x, &y);
    return;
  }
  vec operator+(const vec &a) const { return (vec){x + a.x, y + a.y}; }
  vec operator-(const vec &a) const { return (vec){x - a.x, y - a.y}; }
  vec operator*(const double &k) const { return (vec){k * x, k * y}; }
  vec sca(double t) { return (vec){t * x, t * y}; }
  double dot(vec a) { return x * a.x + y * a.y; }
  double cross(vec a) { return x * a.y - y * a.x; }
  double norm() { return sqrt(x * x + y * y); }
  double norm2() { return x * x + y * y; }
};

bool xsort(vec qwe, vec rty) { return qwe.x < rty.x; }
bool ysort(vec qwe, vec rty) { return qwe.y < rty.y; }

vec pt[500000];
vec ytmp[500000];
vec ymerge[500000];
double ma = 100000000000.0;

double closestpair(int l, int r) {
  if (r - l <= 1)
    return ma;
  int m = l + (r - l) / 2;
  double xm = pt[m].x;
  double res = min(closestpair(l, m), closestpair(m, r));
  double d = sqrt(res);

  int hdmerge = 0, hdtmp = 0, lhd = l, rhd = m;
  vec now;
  for (;;) {
    if (lhd == m && rhd == r)
      break;
    else if (rhd == r)
      now = pt[lhd++];
    else if (lhd == m)
      now = pt[rhd++];
    else if (ysort(pt[lhd], pt[rhd]))
      now = pt[lhd++];
    else
      now = pt[rhd++];

    ymerge[hdmerge++] = now;
    if (abs(now.x - xm) <= res + eps) {
      ytmp[hdtmp++] = now;
      for (int i = hdtmp - 2; i >= 0; --i) {
        if (abs(ytmp[i].y - now.y) >= d - eps)
          break;
        res = min(res, (now - ytmp[i]).norm2());
        d = sqrt(res);
      }
    }
  }
  for (int i = l; i < r; ++i)
    pt[i] = ymerge[i - l];

  return (int)res;
}

signed main(void) {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; ++i)
    pt[i].nextvec();
  sort(pt, pt + n, xsort);
  printf("%d\n", (int)closestpair(0, n));

  return 0;
}