Problem p03798 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <fstream>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;

#define REP(i, m, n) for (int i = (m); i < (int)(n); i++)
#define RREP(i, m, n) for (int i = (int)(n - 1); i >= m; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define aut(r, v) __typeof(v) r = (v)
#define each(it, o) for (aut(it, (o).begin()); it != (o).end(); ++it)
#define reach(it, o) for (aut(it, (o).rbegin()); it != (o).rend(); ++it)
#define fi first
#define se second
#define dump(x) cerr << " [L" << __LINE__ << "] " << #x << " = " << (x) << endl;

template <typename T> string join(vector<T> &v, string del = ", ") {
  stringstream s;
  rep(i, v.size()) s << del << v[i];
  return s.str().substr(del.size());
}
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const pair<T1, T2> &p) {
  return o << "(" << p.first << ", " << p.second << ")";
}
template <typename T> ostream &operator<<(ostream &o, vector<T> &v) {
  if (v.size())
    o << "[" << join(v) << "]";
  return o;
}
template <typename T> ostream &operator<<(ostream &o, vector<vector<T>> &vv) {
  int l = vv.size();
  if (l) {
    rep(i, l) {
      o << (i == 0 ? "[ " : ",\n  ") << vv[i] << (i == l - 1 ? " ]" : "");
    }
  }
  return o;
}
template <typename T> ostream &operator<<(ostream &o, const set<T> &st) {
  vector<T> v(st.begin(), st.end());
  o << "{ " << join(v) << " }";
  return o;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const map<T1, T2> &m) {
  each(p, m) {
    o << (p == m.begin() ? "{ " : ",\n  ") << *p
      << (p == --m.end() ? " }" : "");
  }
  return o;
}

using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;

const double PI = (1 * acos(0.0));
const double INF = 0x3f3f3f3f;
const double INFL = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-9;
const double mod = 1e9 + 7;

inline void finput(string filename) { freopen(filename.c_str(), "r", stdin); }

int N;
vi a, b;

int check(int cur, int pre) {
  a[0] = pre;
  a[1] = cur;
  REP(i, 2, N + 1) a[i] = a[i - 2] ^ a[i - 1] ^ b[i - 1];
  a[N + 1] = a[1];
  a[0] = a[N];
  return !(a[0] ^ a[1] ^ b[1] ^ a[2] || a[N - 1] ^ a[N] ^ b[N] ^ a[N + 1]);
}

int main() {
  ios_base::sync_with_stdio(0);
  finput("./input");

  cin >> N;
  string s;
  cin >> s;

  a = b = vi(N + 2);
  rep(i, N) b[i + 1] = (s[i] == 'o' ? 0 : 1);
  int found = 0;
  if (check(0, 0) || check(1, 0) || check(0, 1) || check(1, 1))
    found = 1;
  if (!found) {
    cout << -1;
  } else {
    REP(i, 1, N + 1) cout << ((a[i] == 0) ? "S" : "W");
  }
  cout << endl;
  return 0;
}


Fixed submission:
#include <algorithm>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <fstream>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;

#define REP(i, m, n) for (int i = (m); i < (int)(n); i++)
#define RREP(i, m, n) for (int i = (int)(n - 1); i >= m; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define aut(r, v) __typeof(v) r = (v)
#define each(it, o) for (aut(it, (o).begin()); it != (o).end(); ++it)
#define reach(it, o) for (aut(it, (o).rbegin()); it != (o).rend(); ++it)
#define fi first
#define se second
#define dump(x) cerr << " [L" << __LINE__ << "] " << #x << " = " << (x) << endl;

template <typename T> string join(vector<T> &v, string del = ", ") {
  stringstream s;
  rep(i, v.size()) s << del << v[i];
  return s.str().substr(del.size());
}
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const pair<T1, T2> &p) {
  return o << "(" << p.first << ", " << p.second << ")";
}
template <typename T> ostream &operator<<(ostream &o, vector<T> &v) {
  if (v.size())
    o << "[" << join(v) << "]";
  return o;
}
template <typename T> ostream &operator<<(ostream &o, vector<vector<T>> &vv) {
  int l = vv.size();
  if (l) {
    rep(i, l) {
      o << (i == 0 ? "[ " : ",\n  ") << vv[i] << (i == l - 1 ? " ]" : "");
    }
  }
  return o;
}
template <typename T> ostream &operator<<(ostream &o, const set<T> &st) {
  vector<T> v(st.begin(), st.end());
  o << "{ " << join(v) << " }";
  return o;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &o, const map<T1, T2> &m) {
  each(p, m) {
    o << (p == m.begin() ? "{ " : ",\n  ") << *p
      << (p == --m.end() ? " }" : "");
  }
  return o;
}

using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;

const double PI = (1 * acos(0.0));
const double INF = 0x3f3f3f3f;
const double INFL = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-9;
const double mod = 1e9 + 7;

inline void finput(string filename) { freopen(filename.c_str(), "r", stdin); }

int N;
vi a, b;

int check(int cur, int pre) {
  a[0] = pre;
  a[1] = cur;
  REP(i, 2, N + 1) a[i] = a[i - 2] ^ a[i - 1] ^ b[i - 1];
  a[N + 1] = a[1];
  a[0] = a[N];
  return !(a[0] ^ a[1] ^ b[1] ^ a[2] || a[N - 1] ^ a[N] ^ b[N] ^ a[N + 1]);
}

int main() {
  ios_base::sync_with_stdio(0);
  // finput("./input");

  cin >> N;
  string s;
  cin >> s;

  a = b = vi(N + 2);
  rep(i, N) b[i + 1] = (s[i] == 'o' ? 0 : 1);
  int found = 0;
  if (check(0, 0) || check(1, 0) || check(0, 1) || check(1, 1))
    found = 1;
  if (!found) {
    cout << -1;
  } else {
    REP(i, 1, N + 1) cout << ((a[i] == 0) ? "S" : "W");
  }
  cout << endl;
  return 0;
}