Problem p00523 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

#define fst first
#define scd second
#define PB push_back
#define MP make_pair
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define omajinai                                                               \
  ios::sync_with_stdio(false);                                                 \
  cin.tie(0)
#define rep(i, x) for (int i = 0; i < (int)(x); ++i)
#define rep1(i, x) for (int i = 1; i <= (int)(x); ++i)

using ll = long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using pii = pair<int, int>;
using vpii = vector<pii>;

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> p) {
  os << "(" << p.fst << ", " << p.scd << ")";
  return os;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> v) {
  rep(i, v.size()) {
    if (i)
      os << ", ";
    os << v[i];
  }
  return os;
}
template <typename T> T &max(T &a, T &b) {
  if (a >= b)
    return a;
  return b;
}
template <typename T> T &min(T &a, T &b) {
  if (a < b)
    return a;
  return b;
}
template <typename T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
int in() {
  int a;
  scanf("%d", &a);
  return a;
}
ll lin() {
  ll a;
  scanf("%lld", &a);
  return a;
}

constexpr int inf = 1e9;
constexpr ll linf = 3e18;
constexpr double eps = 1e-9;

int n;
int A[100010];
ll sum[100010];

bool check(int i, ll x, bool f = false) {
  int nowidx = i;
  ll cur = i == 0 ? 0 : sum[i - 1];

  rep(_, 2) {
    nowidx = lower_bound(sum, sum + i + n, cur + x) - sum;
    if (nowidx == i + n)
      return false;
    cur = sum[nowidx];
  }

  return sum[i + n - 1] - sum[nowidx] >= x;
}

int main() {
  n = in();

  rep(i, n) {
    A[i] = in();
    sum[i] = A[i];
    if (i)
      sum[i] += sum[i - 1];
  }

  rep(i, n - 1) { sum[i + n] = A[i] + sum[i + n - 1]; }

  ll ma = 0;

  rep(i, n) {
    ll l = 0, r = linf;

    rep(_, 65) {
      ll mid = (l + r) / 2;

      (check(i, mid) ? l : r) = mid;
    }

    chmax(ma, l);
  }

  printf("%lld\n", ma);
}

Fixed submission:
#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

#define fst first
#define scd second
#define PB push_back
#define MP make_pair
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define omajinai                                                               \
  ios::sync_with_stdio(false);                                                 \
  cin.tie(0)
#define rep(i, x) for (int i = 0; i < (int)(x); ++i)
#define rep1(i, x) for (int i = 1; i <= (int)(x); ++i)

using ll = long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using pii = pair<int, int>;
using vpii = vector<pii>;

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> p) {
  os << "(" << p.fst << ", " << p.scd << ")";
  return os;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> v) {
  rep(i, v.size()) {
    if (i)
      os << ", ";
    os << v[i];
  }
  return os;
}
template <typename T> T &max(T &a, T &b) {
  if (a >= b)
    return a;
  return b;
}
template <typename T> T &min(T &a, T &b) {
  if (a < b)
    return a;
  return b;
}
template <typename T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
int in() {
  int a;
  scanf("%d", &a);
  return a;
}
ll lin() {
  ll a;
  scanf("%lld", &a);
  return a;
}

constexpr int inf = 1e9;
constexpr ll linf = 3e18;
constexpr double eps = 1e-9;

int n;
int A[100010];
ll sum[200010];

bool check(int i, ll x, bool f = false) {
  int nowidx = i;
  ll cur = i == 0 ? 0 : sum[i - 1];

  rep(_, 2) {
    nowidx = lower_bound(sum, sum + i + n, cur + x) - sum;
    if (nowidx == i + n)
      return false;
    cur = sum[nowidx];
  }

  return sum[i + n - 1] - sum[nowidx] >= x;
}

int main() {
  n = in();

  rep(i, n) {
    A[i] = in();
    sum[i] = A[i];
    if (i)
      sum[i] += sum[i - 1];
  }

  rep(i, n - 1) { sum[i + n] = A[i] + sum[i + n - 1]; }

  ll ma = 0;

  rep(i, n) {
    ll l = 0, r = linf;

    rep(_, 65) {
      ll mid = (l + r) / 2;

      (check(i, mid) ? l : r) = mid;
    }

    chmax(ma, l);
  }

  printf("%lld\n", ma);
}