Problem p03569 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(), a.end()
#define maxs(x, y) (x = max(x, y))
#define mins(x, y) (x = min(x, y))
#define limit(x, l, r) max(l, min(x, r))
#define lims(x, l, r) (x = max(l, min(x, r)))
#define isin(x, l, r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)), x.end())
#define snuke srand((unsigned)clock() + (unsigned)time(NULL));
#define show(x) cout << #x << " = " << x << endl;
#define PQ(T) priority_queue<T, v(T), greater<T>>
#define bn(x) ((1 << x) - 1)
#define dup(x, y) (((x) + (y)-1) / (y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int, int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
inline int in() {
  int x;
  scanf("%d", &x);
  return x;
}
template <typename T> inline istream &operator>>(istream &i, v(T) & v) {
  rep(j, sz(v)) i >> v[j];
  return i;
}
template <typename T> string join(const v(T) & v) {
  stringstream s;
  rep(i, sz(v)) s << ' ' << v[i];
  return s.str().substr(1);
}
template <typename T> inline ostream &operator<<(ostream &o, const v(T) & v) {
  if (sz(v))
    o << join(v);
  return o;
}
template <typename T1, typename T2>
inline istream &operator>>(istream &i, pair<T1, T2> &v) {
  return i >> v.fi >> v.se;
}
template <typename T1, typename T2>
inline ostream &operator<<(ostream &o, const pair<T1, T2> &v) {
  return o << v.fi << "," << v.se;
}
template <typename T> inline ll suma(const v(T) & a) {
  ll res(0);
  for (auto &&x : a)
    res += x;
  return res;
}
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
#define dame                                                                   \
  {                                                                            \
    puts("-1");                                                                \
    return 0;                                                                  \
  }
#define yn                                                                     \
  { puts("Yes"); }                                                             \
  else {                                                                       \
    puts("No");                                                                \
  }
const int MX = 200005;
#define RET(ans)                                                               \
  {                                                                            \
    cout << ans << endl;                                                       \
    return 0;                                                                  \
  }

// 二次元ベクターの基本
/*
vector<vector<int>> dp; // 宣言
dp.resize(n); // 1次元めの要素数決定
dp[i].push_back(int); // プッシュバック
rep(i,n){
    sort(dp[i].begin(),dp[i].end()); // 二次元めを昇順ソート
}
*/

// 整数スキャン（複数）
/*
int x,y,z;
scanf("%d%d%d",&x,&y,&z);

// n個の整数のスキャン
/*
ll a[n] = {};
rep(i,n){
    scanf("%lld",&a[i]);
}
*/

// 文字列スキャン
/*
string s; cin >> s;
*/

// n個の文字列スキャン
/*
vector<string> slist;
rep(i,n){
    string s; cin >> s;
    slist.push_back(s);
}
*/

int main() {
  string s;
  cin >> s;

  string ss;
  rep(i, s.size()) {
    if (s[i] != 'x') {
      ss += s[i];
    }
  }

  rep(i, ss.size() / 2) {
    if (ss[i] != ss[ss.size() - 1 - i]) {
      cout << "-1" << endl;
      return 0;
    }
  }

  int ans = 0;

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

  int l = 0;
  int r = s.size() - 1;

  while (true) {
    if (l == r)
      break;
    if (s[l] == s[r]) {
      l++;
      r--;
    } else if (s[l] == 'x') {
      ans++;
      l++;
    } else {
      ans++;
      r--;
    }
  }

  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(), a.end()
#define maxs(x, y) (x = max(x, y))
#define mins(x, y) (x = min(x, y))
#define limit(x, l, r) max(l, min(x, r))
#define lims(x, l, r) (x = max(l, min(x, r)))
#define isin(x, l, r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)), x.end())
#define snuke srand((unsigned)clock() + (unsigned)time(NULL));
#define show(x) cout << #x << " = " << x << endl;
#define PQ(T) priority_queue<T, v(T), greater<T>>
#define bn(x) ((1 << x) - 1)
#define dup(x, y) (((x) + (y)-1) / (y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int, int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
inline int in() {
  int x;
  scanf("%d", &x);
  return x;
}
template <typename T> inline istream &operator>>(istream &i, v(T) & v) {
  rep(j, sz(v)) i >> v[j];
  return i;
}
template <typename T> string join(const v(T) & v) {
  stringstream s;
  rep(i, sz(v)) s << ' ' << v[i];
  return s.str().substr(1);
}
template <typename T> inline ostream &operator<<(ostream &o, const v(T) & v) {
  if (sz(v))
    o << join(v);
  return o;
}
template <typename T1, typename T2>
inline istream &operator>>(istream &i, pair<T1, T2> &v) {
  return i >> v.fi >> v.se;
}
template <typename T1, typename T2>
inline ostream &operator<<(ostream &o, const pair<T1, T2> &v) {
  return o << v.fi << "," << v.se;
}
template <typename T> inline ll suma(const v(T) & a) {
  ll res(0);
  for (auto &&x : a)
    res += x;
  return res;
}
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
#define dame                                                                   \
  {                                                                            \
    puts("-1");                                                                \
    return 0;                                                                  \
  }
#define yn                                                                     \
  { puts("Yes"); }                                                             \
  else {                                                                       \
    puts("No");                                                                \
  }
const int MX = 200005;
#define RET(ans)                                                               \
  {                                                                            \
    cout << ans << endl;                                                       \
    return 0;                                                                  \
  }

// 二次元ベクターの基本
/*
vector<vector<int>> dp; // 宣言
dp.resize(n); // 1次元めの要素数決定
dp[i].push_back(int); // プッシュバック
rep(i,n){
    sort(dp[i].begin(),dp[i].end()); // 二次元めを昇順ソート
}
*/

// 整数スキャン（複数）
/*
int x,y,z;
scanf("%d%d%d",&x,&y,&z);

// n個の整数のスキャン
/*
ll a[n] = {};
rep(i,n){
    scanf("%lld",&a[i]);
}
*/

// 文字列スキャン
/*
string s; cin >> s;
*/

// n個の文字列スキャン
/*
vector<string> slist;
rep(i,n){
    string s; cin >> s;
    slist.push_back(s);
}
*/

int main() {
  string s;
  cin >> s;

  string ss;
  rep(i, s.size()) {
    if (s[i] != 'x') {
      ss += s[i];
    }
  }

  rep(i, ss.size() / 2) {
    if (ss[i] != ss[ss.size() - 1 - i]) {
      cout << "-1" << endl;
      return 0;
    }
  }

  int ans = 0;

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

  int l = 0;
  int r = s.size() - 1;

  while (true) {
    if (l == r || (s[l] == s[r] && r - l == 1))
      break;
    if (s[l] == s[r]) {
      l++;
      r--;
    } else if (s[l] == 'x') {
      ans++;
      l++;
    } else {
      ans++;
      r--;
    }
  }

  cout << ans << endl;
  return 0;
}