Problem p03950 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define M 200010
inline int read() {
  char ch = getchar();
  int x = 0, f = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-')
      f = -1;
    ch = getchar();
  }
  while ('0' <= ch && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
struct Node {
  int x, y;
  inline bool operator<(const Node &rhs) const {
    return x < rhs.x || (x == rhs.x && y < rhs.y);
  }
} a[M], L[M], R[M];
int lz[M * 4], mx[M * 4];
int w, h, n;
int Ans;
inline void build(int l, int r, int o) {
  lz[o] = 0;
  mx[o] = -1e9;
  if (l == r)
    return;
  int mid = (l + r) / 2;
  build(l, mid, 2 * o);
  build(mid + 1, r, 2 * o + 1);
}
inline void update(int o, int x) {
  mx[o] += x;
  lz[o] += x;
}
inline void pushdown(int l, int r, int o) {
  if (lz[o]) {
    update(2 * o, lz[o]);
    update(2 * o + 1, lz[o]);
    lz[o] = 0;
  }
}
inline void change(int l, int r, int o, int x, int y, int z) {
  if (x <= l && r <= y) {
    update(o, z);
    return;
  }
  pushdown(l, r, o);
  int mid = (l + r) / 2;
  if (x <= mid)
    change(l, mid, 2 * o, x, y, z);
  if (y > mid)
    change(mid + 1, r, 2 * o + 1, x, y, z);
  mx[o] = max(mx[2 * o], mx[2 * o + 1]);
}
inline void Do() {
  build(1, n, 1);
  sort(a + 1, a + n + 1);
  int t1 = 0, t2 = 0;
  for (int i = 1; i < n; ++i) {
    // cerr << i << endl;
    if (a[i].y <= h / 2) {
      // if(i == 3) puts("fuck");
      int lst = i - 1;
      while (t1 && a[i].y > L[t1].y) {
        // if(i == 3) printf("%d %d\n", L[t1].x, lst);
        change(1, n, 1, L[t1].x, lst, L[t1].y - a[i].y);
        lst = L[t1].x - 1;
        --t1;
      }
      if (lst != i - 1)
        L[++t1] = (Node){lst + 1, a[i].y};
    } else {
      int lst = i - 1;
      while (t2 && a[i].y < R[t2].y) {
        change(1, n, 1, R[t2].x, lst, a[i].y - R[t2].y);
        lst = R[t2].x - 1;
        --t2;
      }
      if (lst != i - 1)
        R[++t2] = (Node){lst + 1, a[i].y};
    }
    L[++t1] = (Node){i, 0};
    R[++t2] = (Node){i, h};
    change(1, n, 1, i, i, 1e9 + h - a[i].x);
    Ans = max(Ans, mx[1] + a[i + 1].x);
  }
}
int main() {
  // freopen("a.in", "r", stdin);
  w = read(), h = read(), n = read();
  for (int i = 1; i <= n; ++i) {
    a[i] = (Node){read(), read()};
  }
  a[++n] = (Node){0, 0};
  a[++n] = (Node){w, h};
  // cerr << 233 << endl;
  Do();
  for (int i = 1; i <= n; ++i) {
    swap(a[i].x, a[i].y);
  }
  swap(w, h);
  // cerr << 233 << endl;
  Do();
  printf("%d\n", Ans * 2);
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define M 300010
inline int read() {
  char ch = getchar();
  int x = 0, f = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-')
      f = -1;
    ch = getchar();
  }
  while ('0' <= ch && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
struct Node {
  int x, y;
  inline bool operator<(const Node &rhs) const {
    return x < rhs.x || (x == rhs.x && y < rhs.y);
  }
} a[M], L[M], R[M];
int lz[M * 4], mx[M * 4];
int w, h, n;
int Ans;
inline void build(int l, int r, int o) {
  lz[o] = 0;
  mx[o] = -1e9;
  if (l == r)
    return;
  int mid = (l + r) / 2;
  build(l, mid, 2 * o);
  build(mid + 1, r, 2 * o + 1);
}
inline void update(int o, int x) {
  mx[o] += x;
  lz[o] += x;
}
inline void pushdown(int l, int r, int o) {
  if (lz[o]) {
    update(2 * o, lz[o]);
    update(2 * o + 1, lz[o]);
    lz[o] = 0;
  }
}
inline void change(int l, int r, int o, int x, int y, int z) {
  if (x <= l && r <= y) {
    update(o, z);
    return;
  }
  pushdown(l, r, o);
  int mid = (l + r) / 2;
  if (x <= mid)
    change(l, mid, 2 * o, x, y, z);
  if (y > mid)
    change(mid + 1, r, 2 * o + 1, x, y, z);
  mx[o] = max(mx[2 * o], mx[2 * o + 1]);
}
inline void Do() {
  build(1, n, 1);
  sort(a + 1, a + n + 1);
  int t1 = 0, t2 = 0;
  for (int i = 1; i < n; ++i) {
    // cerr << i << endl;
    if (a[i].y <= h / 2) {
      // if(i == 3) puts("fuck");
      int lst = i - 1;
      while (t1 && a[i].y > L[t1].y) {
        // if(i == 3) printf("%d %d\n", L[t1].x, lst);
        change(1, n, 1, L[t1].x, lst, L[t1].y - a[i].y);
        lst = L[t1].x - 1;
        --t1;
      }
      if (lst != i - 1)
        L[++t1] = (Node){lst + 1, a[i].y};
    } else {
      int lst = i - 1;
      while (t2 && a[i].y < R[t2].y) {
        change(1, n, 1, R[t2].x, lst, a[i].y - R[t2].y);
        lst = R[t2].x - 1;
        --t2;
      }
      if (lst != i - 1)
        R[++t2] = (Node){lst + 1, a[i].y};
    }
    L[++t1] = (Node){i, 0};
    R[++t2] = (Node){i, h};
    change(1, n, 1, i, i, 1e9 + h - a[i].x);
    Ans = max(Ans, mx[1] + a[i + 1].x);
  }
}
int main() {
  // freopen("a.in", "r", stdin);
  w = read(), h = read(), n = read();
  for (int i = 1; i <= n; ++i) {
    a[i] = (Node){read(), read()};
  }
  a[++n] = (Node){0, 0};
  a[++n] = (Node){w, h};
  // cerr << 233 << endl;
  Do();
  for (int i = 1; i <= n; ++i) {
    swap(a[i].x, a[i].y);
  }
  swap(w, h);
  // cerr << 233 << endl;
  Do();
  printf("%d\n", Ans * 2);
}