Problem p03569 (C++)

Buggy submission (status: Runtime Error):
#pragma GCC optimize("O3")
#pragma GCC target("avx")
#include "bits/stdc++.h" // define macro "/D__MAI"

using namespace std;
typedef long long int ll;

#define xprintf(fmt, ...) fprintf(stderr, fmt, __VA_ARGS__)
#define debugv(v)                                                              \
  {                                                                            \
    printf("L%d %s > ", __LINE__, #v);                                         \
    for (auto e : v) {                                                         \
      cout << e << " ";                                                        \
    }                                                                          \
    cout << endl;                                                              \
  }
#define debuga(m, w)                                                           \
  {                                                                            \
    printf("L%d %s > ", __LINE__, #m);                                         \
    for (int x = 0; x < (w); x++) {                                            \
      cout << (m)[x] << " ";                                                   \
    }                                                                          \
    cout << endl;                                                              \
  }
#define debugaa(m, h, w)                                                       \
  {                                                                            \
    printf("L%d %s >\n", __LINE__, #m);                                        \
    for (int y = 0; y < (h); y++) {                                            \
      for (int x = 0; x < (w); x++) {                                          \
        cout << (m)[y][x] << " ";                                              \
      }                                                                        \
      cout << endl;                                                            \
    }                                                                          \
  }
#define ALL(v) (v).begin(), (v).end()
#define repeat(cnt, l) for (auto cnt = 0ll; (cnt) < (l); ++(cnt))
#define iterate(cnt, b, e) for (auto cnt = (b); (cnt) != (e); ++(cnt))
#define MD 1000000007ll
#define PI 3.1415926535897932384626433832795
#define EPS 1e-12
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const pair<T1, T2> p) {
  o << "(" << p.first << ":" << p.second << ")";
  return o;
}
template <typename iterator>
inline size_t argmin(iterator begin, iterator end) {
  return distance(begin, min_element(begin, end));
}
template <typename iterator>
inline size_t argmax(iterator begin, iterator end) {
  return distance(begin, max_element(begin, end));
}
template <typename T> T &maxset(T &to, const T &val) {
  return to = max(to, val);
}
template <typename T> T &minset(T &to, const T &val) {
  return to = min(to, val);
}

mt19937_64 randdev(8901016);
inline ll rand_range(ll l, ll h) {
  return uniform_int_distribution<ll>(l, h)(randdev);
}

#ifdef __MAI
#define getchar_unlocked getchar
#define putchar_unlocked putchar
#endif
#ifdef __VSCC
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#endif
namespace {
#define isvisiblechar(c) (0x21 <= (c) && (c) <= 0x7E)
class MaiScanner {
public:
  template <typename T> void input_integer(T &var) {
    var = 0;
    T sign = 1;
    int cc = getchar_unlocked();
    for (; cc < '0' || '9' < cc; cc = getchar_unlocked())
      if (cc == '-')
        sign = -1;
    for (; '0' <= cc && cc <= '9'; cc = getchar_unlocked())
      var = (var << 3) + (var << 1) + cc - '0';
    var = var * sign;
  }
  inline int c() { return getchar_unlocked(); }
  inline MaiScanner &operator>>(int &var) {
    input_integer<int>(var);
    return *this;
  }
  inline MaiScanner &operator>>(long long &var) {
    input_integer<long long>(var);
    return *this;
  }
  inline MaiScanner &operator>>(string &var) {
    int cc = getchar_unlocked();
    for (; !isvisiblechar(cc); cc = getchar_unlocked())
      ;
    for (; isvisiblechar(cc); cc = getchar_unlocked())
      var.push_back(cc);
    return *this;
  }
  template <typename IT> void in(IT begin, IT end) {
    for (auto it = begin; it != end; ++it)
      *this >> *it;
  }
};
} // namespace
MaiScanner scanner;

// オンライン回文判定
bool iskaibun(const string &str) {
  int left = 0;
  int right = str.size() - 1;
  while (left < right) {
    if (str[left] != str[right])
      return false;
    ++left;
    --right;
  }
  return true;
}

//

ll m, n, kei;

// ある地点iを中心としたとき，左のx以外の文字の数と→のx以外の文字の数が一致していればおｋ

string str;
string str2;
vector<int> ptr;

int chrcnt[100010][30];

int main() {

  scanner >> str;

  n = str.size();

  repeat(i, n) {
    if (str[i] != 'x')
      str2.push_back(str[i]), ptr.push_back(i);
  }

  if (!iskaibun(str2)) {
    cout << -1 << endl;
    return 0;
  }

  int left;
  int right;
  if (str2.size() % 2 == 1) {
    // 奇数！
    int center = ptr[str2.size() / 2];
    left = center;
    right = center;
  } else {
    left = ptr[str2.size() / 2 - 1];
    right = ptr[str2.size() / 2];
  }

  int cnt = 0;
  while (0 <= left && right < n) {
    // cout << left << " " << right << endl;
    if (str[left] == str[right] && str[left] != 'x') {
      --left;
      ++right;
    } else {
      cnt += str[left] != str[right];
      if (str[left] == 'x')
        --left;
      if (str[right] == 'x')
        ++right;
    }
  }
  while (0 <= left)
    --left, ++cnt;
  while (right < n)
    ++right, ++cnt;
  cout << cnt << endl;

  // repeat(i, n) {
  //     chrcnt[i][str[i] - 'a'] = 1;
  // }
  //
  // repeat(i, n) {
  //     repeat(j, 28) {
  //         chrcnt[i + 1][j] += chrcnt[i][j];
  //     }
  // }

  return 0;
}

Fixed submission:
#pragma GCC optimize("O3")
#pragma GCC target("avx")
#include "bits/stdc++.h" // define macro "/D__MAI"

using namespace std;
typedef long long int ll;

#define xprintf(fmt, ...) fprintf(stderr, fmt, __VA_ARGS__)
#define debugv(v)                                                              \
  {                                                                            \
    printf("L%d %s > ", __LINE__, #v);                                         \
    for (auto e : v) {                                                         \
      cout << e << " ";                                                        \
    }                                                                          \
    cout << endl;                                                              \
  }
#define debuga(m, w)                                                           \
  {                                                                            \
    printf("L%d %s > ", __LINE__, #m);                                         \
    for (int x = 0; x < (w); x++) {                                            \
      cout << (m)[x] << " ";                                                   \
    }                                                                          \
    cout << endl;                                                              \
  }
#define debugaa(m, h, w)                                                       \
  {                                                                            \
    printf("L%d %s >\n", __LINE__, #m);                                        \
    for (int y = 0; y < (h); y++) {                                            \
      for (int x = 0; x < (w); x++) {                                          \
        cout << (m)[y][x] << " ";                                              \
      }                                                                        \
      cout << endl;                                                            \
    }                                                                          \
  }
#define ALL(v) (v).begin(), (v).end()
#define repeat(cnt, l) for (auto cnt = 0ll; (cnt) < (l); ++(cnt))
#define iterate(cnt, b, e) for (auto cnt = (b); (cnt) != (e); ++(cnt))
#define MD 1000000007ll
#define PI 3.1415926535897932384626433832795
#define EPS 1e-12
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const pair<T1, T2> p) {
  o << "(" << p.first << ":" << p.second << ")";
  return o;
}
template <typename iterator>
inline size_t argmin(iterator begin, iterator end) {
  return distance(begin, min_element(begin, end));
}
template <typename iterator>
inline size_t argmax(iterator begin, iterator end) {
  return distance(begin, max_element(begin, end));
}
template <typename T> T &maxset(T &to, const T &val) {
  return to = max(to, val);
}
template <typename T> T &minset(T &to, const T &val) {
  return to = min(to, val);
}

mt19937_64 randdev(8901016);
inline ll rand_range(ll l, ll h) {
  return uniform_int_distribution<ll>(l, h)(randdev);
}

#ifdef __MAI
#define getchar_unlocked getchar
#define putchar_unlocked putchar
#endif
#ifdef __VSCC
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#endif
namespace {
#define isvisiblechar(c) (0x21 <= (c) && (c) <= 0x7E)
class MaiScanner {
public:
  template <typename T> void input_integer(T &var) {
    var = 0;
    T sign = 1;
    int cc = getchar_unlocked();
    for (; cc < '0' || '9' < cc; cc = getchar_unlocked())
      if (cc == '-')
        sign = -1;
    for (; '0' <= cc && cc <= '9'; cc = getchar_unlocked())
      var = (var << 3) + (var << 1) + cc - '0';
    var = var * sign;
  }
  inline int c() { return getchar_unlocked(); }
  inline MaiScanner &operator>>(int &var) {
    input_integer<int>(var);
    return *this;
  }
  inline MaiScanner &operator>>(long long &var) {
    input_integer<long long>(var);
    return *this;
  }
  inline MaiScanner &operator>>(string &var) {
    int cc = getchar_unlocked();
    for (; !isvisiblechar(cc); cc = getchar_unlocked())
      ;
    for (; isvisiblechar(cc); cc = getchar_unlocked())
      var.push_back(cc);
    return *this;
  }
  template <typename IT> void in(IT begin, IT end) {
    for (auto it = begin; it != end; ++it)
      *this >> *it;
  }
};
} // namespace
MaiScanner scanner;

// オンライン回文判定
bool iskaibun(const string &str) {
  int left = 0;
  int right = str.size() - 1;
  while (left < right) {
    if (str[left] != str[right])
      return false;
    ++left;
    --right;
  }
  return true;
}

//

ll m, n, kei;

// ある地点iを中心としたとき，左のx以外の文字の数と→のx以外の文字の数が一致していればおｋ

string str;
string str2;
vector<int> ptr;

int chrcnt[100010][30];

int main() {

  scanner >> str;

  n = str.size();

  repeat(i, n) {
    if (str[i] != 'x')
      str2.push_back(str[i]), ptr.push_back(i);
  }

  if (str2.size() == 0) {
    cout << 0 << endl;
    return 0;
  }

  if (!iskaibun(str2)) {
    cout << -1 << endl;
    return 0;
  }

  int left;
  int right;
  if (str2.size() % 2 == 1) {
    // 奇数！
    int center = ptr[str2.size() / 2];
    left = center;
    right = center;
  } else {
    left = ptr[str2.size() / 2 - 1];
    right = ptr[str2.size() / 2];
  }

  int cnt = 0;
  while (0 <= left && right < n) {
    // cout << left << " " << right << endl;
    if (str[left] == str[right] && str[left] != 'x') {
      --left;
      ++right;
    } else {
      cnt += str[left] != str[right];
      if (str[left] == 'x')
        --left;
      if (str[right] == 'x')
        ++right;
    }
  }
  while (0 <= left)
    --left, ++cnt;
  while (right < n)
    ++right, ++cnt;
  cout << cnt << endl;

  // repeat(i, n) {
  //     chrcnt[i][str[i] - 'a'] = 1;
  // }
  //
  // repeat(i, n) {
  //     repeat(j, 28) {
  //         chrcnt[i + 1][j] += chrcnt[i][j];
  //     }
  // }

  return 0;
}