Problem p03460 (C++)

Buggy submission (status: Time Limit Exceeded):
// #include <bits/stdc++.h>
#include <bitset>
#include <ctime>
#include <iostream>
#include <map>
#include <unordered_map>
#include <vector>

using namespace std;

#define rep(i, n) REP(i, 0, n)
#define REP(i, s, e) for (int i = (s); i < (int)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for (int i = (int)(s - 1); i >= (int)(e); i--)
#define pb push_back
#define all(r) r.begin(), r.end()
#define rall(r) r.rbegin(), r.rend()
#define fi first
#define se second

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int INF = 1e9;
const ll MOD = 1e9 + 7;
double EPS = 1e-8;

const int MAX_N = 1e5 + 10;
int x[MAX_N], y[MAX_N], z[MAX_N];
const int MAX_K = 1001;
const int MAX_S = 3 * MAX_K + 5;
int s[MAX_S];
const int BASE = MAX_K + 1;
int prvX[MAX_N];
int sxs[MAX_N];

typedef bitset<MAX_N> BS;
unordered_map<BS, int> mp;

void f(int n, int k) {
  BS bs;
  rep(sx, k) {
    rep(i, n) bs[i] = (((x[i] + sx) / k) & 1);
    mp[bs] = sx;
  }
}

int main() {
  int n, k;
  cin >> n >> k;
  rep(i, n) {
    char c;
    cin >> x[i] >> y[i] >> c;
    if (c == 'W')
      z[i] = 1;
    else
      z[i] = 0;
  }
  f(n, k);
  srand((unsigned)time(NULL));
  while (mp.size() > 500) {
    int x = rand() % mp.size();
    auto it = mp.begin();
    rep(i, x)++ it;
    mp.erase(it);
  }
  int cnt = 0;
  for (auto &p : mp)
    sxs[cnt++] = p.se;
  int ans = 0;
  rep(i, cnt) rep(c, 2) {
    int sx = sxs[i];
    // int idxmin = INF, idxmax = -INF;
    rep(i, n) {
      int X = (x[i] + sx) / k;
      int Y = y[i] / k;
      int ymin = Y * k - y[i];
      int ymax = ymin + k;
      if ((X + Y + c) % 2 == z[i]) {
        ymin += k;
        ymax += k;
      }
      s[ymin + BASE] += 1;
      s[ymax + BASE] -= 1;
      // idxmin = min(idxmin, ymin + BASE);
      // idxmax = max(idxmax, ymax + BASE);
    }
    // REP(i, idxmin, idxmax) s[i + 1] += s[i];
    // REP(i, idxmin, idxmax + 1) {
    //     ans = max(ans, s[i]);
    //     s[i] = 0;
    // }
    rep(i, MAX_S - 1) s[i + 1] += s[i];
    rep(i, MAX_S) {
      ans = max(ans, s[i]);
      s[i] = 0;
    }
  }
  cout << ans << endl;
  return 0;
}

Fixed submission:
// #include <bits/stdc++.h>
#include <bitset>
#include <ctime>
#include <iostream>
#include <map>
#include <unordered_map>
#include <vector>

using namespace std;

#define rep(i, n) REP(i, 0, n)
#define REP(i, s, e) for (int i = (s); i < (int)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for (int i = (int)(s - 1); i >= (int)(e); i--)
#define pb push_back
#define all(r) r.begin(), r.end()
#define rall(r) r.rbegin(), r.rend()
#define fi first
#define se second

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int INF = 1e9;
const ll MOD = 1e9 + 7;
double EPS = 1e-8;

const int MAX_N = 1e5 + 10;
int x[MAX_N], y[MAX_N], z[MAX_N];
const int MAX_K = 1001;
const int MAX_S = 3 * MAX_K + 5;
int s[MAX_S];
const int BASE = MAX_K + 1;
int prvX[MAX_N];
int sxs[MAX_N];

typedef bitset<MAX_N> BS;
unordered_map<BS, int> mp;

void f(int n, int k) {
  BS bs;
  rep(sx, k) {
    rep(i, n) bs[i] = (((x[i] + sx) / k) & 1);
    mp[bs] = sx;
  }
}

int main() {
  int n, k;
  cin >> n >> k;
  rep(i, n) {
    char c;
    cin >> x[i] >> y[i] >> c;
    if (c == 'W')
      z[i] = 1;
    else
      z[i] = 0;
  }
  f(n, k);
  srand((unsigned)time(NULL));
  while (mp.size() > 410) {
    int x = rand() % mp.size();
    auto it = mp.begin();
    rep(i, x)++ it;
    mp.erase(it);
  }
  int cnt = 0;
  for (auto &p : mp)
    sxs[cnt++] = p.se;
  int ans = 0;
  rep(i, cnt) rep(c, 2) {
    int sx = sxs[i];
    // int idxmin = INF, idxmax = -INF;
    rep(i, n) {
      int X = (x[i] + sx) / k;
      int Y = y[i] / k;
      int ymin = Y * k - y[i];
      int ymax = ymin + k;
      if ((X + Y + c) % 2 == z[i]) {
        ymin += k;
        ymax += k;
      }
      s[ymin + BASE] += 1;
      s[ymax + BASE] -= 1;
      // idxmin = min(idxmin, ymin + BASE);
      // idxmax = max(idxmax, ymax + BASE);
    }
    // REP(i, idxmin, idxmax) s[i + 1] += s[i];
    // REP(i, idxmin, idxmax + 1) {
    //     ans = max(ans, s[i]);
    //     s[i] = 0;
    // }
    rep(i, MAX_S - 1) s[i + 1] += s[i];
    rep(i, MAX_S) {
      ans = max(ans, s[i]);
      s[i] = 0;
    }
  }
  cout << ans << endl;
  return 0;
}