Problem p03798 (C++)

Buggy submission (status: Runtime Error):
#include "bits/stdc++.h"
using namespace std;
///////////////////////////////////////////
const long long int INF = 1LL << 60;
const long long int Mod = 1000000007;
using ll = long long int;
using ci = const int;
using vi = vector<int>;
using Vi = vector<long long int>;
using P = pair<int, int>;
using PLL = pair<ll, ll>;
using matrix = vector<vector<ll>>;
#define pb(x) push_back(x)
#define mp(x, y) make_pair(x, y)
#define all(x) (x).begin(), (x).end()
#define rep(i, N) for (ll i = 0; i < (ll)N; i++)
#define repi(i, a, b) for (ll i = ll(a); i < ll(b); ++i)
template <class T> bool chmax(T &former, const T &b) {
  if (former < b) {
    former = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &former, const T &b) {
  if (b < former) {
    former = b;
    return true;
  }
  return false;
}
template <class T> T sqar(T x) { return x * x; } // sqrt(x)は平方根;
#define Sort(v)                                                                \
  std::sort(v.begin(), v.end(),                                                \
            std::greater<decltype(v[0])>()) // 降順でVをソート
template <class T> inline void princ(T x) { cout << x << " "; };
template <class T> inline void print(T x) { cout << x << "\n"; };
template <class T> inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T> inline void YES(T condition) {
  if (condition)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
///////////////////////////////////////////////////////////////////////////////////

ll n;
string s;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(30);
  string ans;
  cin >> n >> s;
  Vi v(n + 1, -1);
  rep(i, 2) {
    rep(j, 2) {
      v[0] = i;
      v[1] = j;
      v.resize(n + 1, -1);
      for (ll k = 1; k <= n; k++) {
        if (v[k] == 0) {
          if (s[k] == 'o') {
            v[k + 1] = v[k - 1];
          } else {
            v[k + 1] = (v[k - 1] + 1) % 2;
          }
        } else { // ookami
          if (s[k] == 'x') {
            v[k + 1] = v[k - 1];
          } else {
            v[k + 1] = (v[k - 1] + 1) % 2;
          }
        }
      }
      if (v[0] != v[n])
        continue;
      if (v[0] == 0) {
        if (s[0] == 'o') {
          if (v[1] != v[n - 1])
            continue;
        } else {
          if (v[1] == v[n - 1])
            continue;
        }
      } else {
        if (s[0] == 'o') {
          if (v[1] == v[n - 1])
            continue;
        } else {
          if (v[1] != v[n - 1])
            continue;
        }
      }

      rep(i, n) {
        if (v[i] == 0)
          ans.pb('S');
        else
          ans.pb('W');
      }
      print(ans);
      return 0;
    }
  }
  print(-1);
  return 0;
}

Fixed submission:
#include "bits/stdc++.h"
using namespace std;
///////////////////////////////////////////
const long long int INF = 1LL << 60;
const long long int Mod = 1000000007;
using ll = long long int;
using ci = const int;
using vi = vector<int>;
using Vi = vector<long long int>;
using P = pair<int, int>;
using PLL = pair<ll, ll>;
using matrix = vector<vector<ll>>;
#define pb(x) push_back(x)
#define mp(x, y) make_pair(x, y)
#define all(x) (x).begin(), (x).end()
#define rep(i, N) for (ll i = 0; i < (ll)N; i++)
#define repi(i, a, b) for (ll i = ll(a); i < ll(b); ++i)
template <class T> bool chmax(T &former, const T &b) {
  if (former < b) {
    former = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &former, const T &b) {
  if (b < former) {
    former = b;
    return true;
  }
  return false;
}
template <class T> T sqar(T x) { return x * x; } // sqrt(x)は平方根;
#define Sort(v)                                                                \
  std::sort(v.begin(), v.end(),                                                \
            std::greater<decltype(v[0])>()) // 降順でVをソート
template <class T> inline void princ(T x) { cout << x << " "; };
template <class T> inline void print(T x) { cout << x << "\n"; };
template <class T> inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T> inline void YES(T condition) {
  if (condition)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
///////////////////////////////////////////////////////////////////////////////////

ll n;
string s;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(30);
  string ans;
  cin >> n >> s;
  Vi v(n + 1, -1);
  rep(i, 2) {
    rep(j, 2) {
      v[0] = i;
      v[1] = j;
      v.resize(n + 1, -1);
      for (ll k = 1; k < n; k++) {
        if (v[k] == 0) {
          if (s[k] == 'o') {
            v[k + 1] = v[k - 1];
          } else {
            v[k + 1] = (v[k - 1] + 1) % 2;
          }
        } else { // ookami
          if (s[k] == 'x') {
            v[k + 1] = v[k - 1];
          } else {
            v[k + 1] = (v[k - 1] + 1) % 2;
          }
        }
      }
      if (v[0] != v[n])
        continue;
      if (v[0] == 0) {
        if (s[0] == 'o') {
          if (v[1] != v[n - 1])
            continue;
        } else {
          if (v[1] == v[n - 1])
            continue;
        }
      } else {
        if (s[0] == 'o') {
          if (v[1] == v[n - 1])
            continue;
        } else {
          if (v[1] != v[n - 1])
            continue;
        }
      }

      rep(i, n) {
        if (v[i] == 0)
          ans.pb('S');
        else
          ans.pb('W');
      }
      print(ans);
      return 0;
    }
  }
  print(-1);
  return 0;
}