Problem p02673 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define LL long long
#define db long double
using namespace std;
template <typename T> void read(T &x) {
  x = 0;
  int f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-')
      f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  x *= f;
}
inline void write(int x) {
  if (x > 9)
    write(x / 10);
  putchar(x % 10 + '0');
}

const int N = 200050;
int n;
LL a[N], b[N], tmp[N], preb[N], prebi[N];
inline void init() {
  int i, id = 1;
  read(n);
  for (i = 1; i <= n; ++i)
    read(a[i]);
  for (i = 1; i <= n; ++i)
    read(b[i]);
  for (i = 1; i <= n; ++i)
    if (a[i] > a[id])
      id = i;
  if (id != 1) {
    --id;
    for (i = 1; i <= n; ++i)
      tmp[i] = a[i];
    for (i = 1; i <= n; ++i)
      a[(i <= id ? i + n - id : i - id)] = tmp[i];
    for (i = 1; i <= n; ++i)
      tmp[i] = b[i];
    for (i = 1; i <= n; ++i)
      b[(i <= id ? i + n - id : i - id)] = tmp[i];
  }
  a[n + 1] = a[1], b[n + 1] = b[1];
  a[0] = a[n], b[0] = b[n];
  for (i = 1; i <= n + 1; ++i)
    preb[i] = preb[i - 1] + b[i], prebi[i] = prebi[i - 1] + b[i] * i;
}
inline db calc(int l, int r, int m) {
  if (l == r)
    return a[l];
  db k = r - l,
     b = (db)(2.0) * (r) * (preb[r - 1] - preb[l]) -
         (db)(2.0) * (prebi[r - 1] - prebi[l]),
     y = a[r] - a[l], x = (y - b) / k;
  return a[l] + (m - l) * x + (db)(2.0) * (m) * (preb[m - 1] - preb[l]) -
         (db)(2.0) * (prebi[m - 1] - prebi[l]);
}
int stk[N], top;
db ans, mx[N];
int main() {
  init();
  int i, j, ll, rr;
  stk[0] = 1;
  for (i = 1; i <= n + 1; ++i) {
    while (top && calc(stk[top - 1], i, stk[top]) >=
                      calc(stk[top - 1], stk[top], stk[top]))
      --top;
    stk[++top] = i;
  }
  for (ll = 1; ll <= n + 1; ++ll)
    for (rr = ll; rr <= n + 1; ++rr)
      for (i = ll; i <= rr; ++i)
        mx[i] = max(mx[i], calc(ll, rr, i));
  for (i = 0; i <= top; ++i) {
    ll = stk[i], rr = stk[i + 1];
    for (j = ll; j <= rr && j <= n; ++j)
      mx[j] = max(mx[j], calc(ll, rr, j));
  }
  for (i = 1; i <= n; ++i)
    ans += mx[i]; //,cerr << i << ' ' << mx[i] << '\n';
  ans /= n;
  cout << fixed << setprecision(20) << ans << '\n';
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define LL long long
#define db long double
using namespace std;
template <typename T> void read(T &x) {
  x = 0;
  int f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-')
      f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  x *= f;
}
inline void write(int x) {
  if (x > 9)
    write(x / 10);
  putchar(x % 10 + '0');
}

const int N = 200050;
int n;
LL a[N], b[N], tmp[N], preb[N], prebi[N];
inline void init() {
  int i, id = 1;
  read(n);
  for (i = 1; i <= n; ++i)
    read(a[i]);
  for (i = 1; i <= n; ++i)
    read(b[i]);
  for (i = 1; i <= n; ++i)
    if (a[i] > a[id])
      id = i;
  if (id != 1) {
    --id;
    for (i = 1; i <= n; ++i)
      tmp[i] = a[i];
    for (i = 1; i <= n; ++i)
      a[(i <= id ? i + n - id : i - id)] = tmp[i];
    for (i = 1; i <= n; ++i)
      tmp[i] = b[i];
    for (i = 1; i <= n; ++i)
      b[(i <= id ? i + n - id : i - id)] = tmp[i];
  }
  a[n + 1] = a[1], b[n + 1] = b[1];
  a[0] = a[n], b[0] = b[n];
  for (i = 1; i <= n + 1; ++i)
    preb[i] = preb[i - 1] + b[i], prebi[i] = prebi[i - 1] + b[i] * i;
}
inline db calc(int l, int r, int m) {
  if (l == r)
    return a[l];
  db k = r - l,
     b = (db)(2.0) * (r) * (preb[r - 1] - preb[l]) -
         (db)(2.0) * (prebi[r - 1] - prebi[l]),
     y = a[r] - a[l], x = (y - b) / k;
  return a[l] + (m - l) * x + (db)(2.0) * (m) * (preb[m - 1] - preb[l]) -
         (db)(2.0) * (prebi[m - 1] - prebi[l]);
}
int stk[N], top;
db ans, mx[N];
int main() {
  init();
  int i, j, ll, rr;
  stk[0] = 1;
  for (i = 1; i <= n + 1; ++i) {
    while (top && calc(stk[top - 1], i, stk[top]) >=
                      calc(stk[top - 1], stk[top], stk[top]))
      --top;
    stk[++top] = i;
  }
  for (i = 0; i <= top; ++i) {
    ll = stk[i], rr = stk[i + 1];
    for (j = ll; j <= rr && j <= n; ++j)
      mx[j] = max(mx[j], calc(ll, rr, j));
  }
  for (i = 1; i <= n; ++i)
    ans += mx[i]; //,cerr << i << ' ' << mx[i] << '\n';
  ans /= n;
  cout << fixed << setprecision(20) << ans << '\n';
  return 0;
}