Problem p03950 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
const int maxn = 3e5 + 911;
struct lpl {
  int x, y;
  bool operator<(const lpl &a) const { return x < a.x; }
} p[maxn];
struct Segment_Tree {
  int w, tag;
} tr[maxn * 2];
int w, h, n, Y[maxn];
int st[2][maxn], top[2], ans;

inline int ls(int t) { return (t << 1); }
inline int rs(int t) { return (t << 1 | 1); }

inline void update(int t) { tr[t].w = max(tr[ls(t)].w, tr[rs(t)].w); }

inline void pushdown(int t) {
  if (!tr[t].tag)
    return;
  tr[ls(t)].w += tr[t].tag;
  tr[ls(t)].tag += tr[t].tag;
  tr[rs(t)].w += tr[t].tag;
  tr[rs(t)].tag += tr[t].tag;
  tr[t].tag = 0;
}

void modify(int k, int l, int r, int x, int y, int val) {
  if (l >= x && r <= y) {
    tr[k].w += val, tr[k].tag += val;
    return;
  }
  int mid = (l + r) >> 1;
  pushdown(k);
  if (x <= mid)
    modify(ls(k), l, mid, x, y, val);
  if (y > mid)
    modify(rs(k), mid + 1, r, x, y, val);
  update(k);
}

int query(int k, int l, int r, int x) {
  if (l == r)
    return tr[k].w;
  int mid = (l + r) >> 1;
  pushdown(k);
  if (x <= mid)
    return query(ls(k), l, mid, x);
  return query(rs(k), mid + 1, r, x);
}

inline void workk() {
  sort(p + 2, p + n + 1);
  top[0] = top[1] = st[0][0] = st[1][0] = 0;
  modify(1, 1, n, 1, n, (h << 1));
  for (int i = 2; i <= n; ++i) {
    modify(1, 1, n, 1, i - 1, (p[i].x - p[i - 1].x) << 1);
    ans = max(ans, tr[1].w);
    if (p[i].y >= (h >> 1)) {
      modify(1, 1, n, st[0][top[0]], i - 1, (p[i].y - h) << 1);
      while (top[0] && p[st[0][top[0]]].y >= p[i].y) {
        modify(1, 1, n, st[0][top[0] - 1], st[0][top[0]] - 1,
               (p[i].y - p[st[0][top[0]]].y) << 1);
        top[0]--;
      }
      st[0][++top[0]] = i;
    } else {
      modify(1, 1, n, st[1][top[1]], i - 1, -p[i].y << 1);
      while (top[1] && p[st[1][top[1]]].y <= p[i].y) {
        modify(1, 1, n, st[1][top[1] - 1], st[1][top[1]] - 1,
               (p[st[1][top[1]]].y - p[i].y) << 1);
        top[1]--;
      }
      st[1][++top[1]] = i;
    }
  }
  memset(tr, 0, sizeof(tr));
}

int main() {
  scanf("%d%d%d", &w, &h, &n);
  ++n;
  for (int i = 2; i <= n; ++i)
    scanf("%d%d", &p[i].x, &p[i].y);
  p[++n] = (lpl){w, h};
  workk();
  swap(w, h);
  for (int i = 1; i <= n; ++i)
    swap(p[i].x, p[i].y);
  workk();
  cout << ans;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
const int maxn = 3e5 + 911;
struct lpl {
  int x, y;
  bool operator<(const lpl &a) const { return x < a.x; }
} p[maxn];
struct Segment_Tree {
  int w, tag;
} tr[maxn * 4];
int w, h, n, Y[maxn];
int st[2][maxn], top[2], ans;

inline int ls(int t) { return (t << 1); }
inline int rs(int t) { return (t << 1 | 1); }

inline void update(int t) { tr[t].w = max(tr[ls(t)].w, tr[rs(t)].w); }

inline void pushdown(int t) {
  if (!tr[t].tag)
    return;
  tr[ls(t)].w += tr[t].tag;
  tr[ls(t)].tag += tr[t].tag;
  tr[rs(t)].w += tr[t].tag;
  tr[rs(t)].tag += tr[t].tag;
  tr[t].tag = 0;
}

void modify(int k, int l, int r, int x, int y, int val) {
  if (l >= x && r <= y) {
    tr[k].w += val, tr[k].tag += val;
    return;
  }
  int mid = (l + r) >> 1;
  pushdown(k);
  if (x <= mid)
    modify(ls(k), l, mid, x, y, val);
  if (y > mid)
    modify(rs(k), mid + 1, r, x, y, val);
  update(k);
}

int query(int k, int l, int r, int x) {
  if (l == r)
    return tr[k].w;
  int mid = (l + r) >> 1;
  pushdown(k);
  if (x <= mid)
    return query(ls(k), l, mid, x);
  return query(rs(k), mid + 1, r, x);
}

inline void workk() {
  sort(p + 2, p + n + 1);
  top[0] = top[1] = st[0][0] = st[1][0] = 0;
  modify(1, 1, n, 1, n, (h << 1));
  for (int i = 2; i <= n; ++i) {
    modify(1, 1, n, 1, i - 1, (p[i].x - p[i - 1].x) << 1);
    ans = max(ans, tr[1].w);
    if (p[i].y >= (h >> 1)) {
      modify(1, 1, n, st[0][top[0]], i - 1, (p[i].y - h) << 1);
      while (top[0] && p[st[0][top[0]]].y >= p[i].y) {
        modify(1, 1, n, st[0][top[0] - 1], st[0][top[0]] - 1,
               (p[i].y - p[st[0][top[0]]].y) << 1);
        top[0]--;
      }
      st[0][++top[0]] = i;
    } else {
      modify(1, 1, n, st[1][top[1]], i - 1, -p[i].y << 1);
      while (top[1] && p[st[1][top[1]]].y <= p[i].y) {
        modify(1, 1, n, st[1][top[1] - 1], st[1][top[1]] - 1,
               (p[st[1][top[1]]].y - p[i].y) << 1);
        top[1]--;
      }
      st[1][++top[1]] = i;
    }
  }
  memset(tr, 0, sizeof(tr));
}

int main() {
  scanf("%d%d%d", &w, &h, &n);
  ++n;
  for (int i = 2; i <= n; ++i)
    scanf("%d%d", &p[i].x, &p[i].y);
  p[++n] = (lpl){w, h};
  workk();
  swap(w, h);
  for (int i = 1; i <= n; ++i)
    swap(p[i].x, p[i].y);
  workk();
  cout << ans;
  return 0;
}