Problem p03800 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

typedef long long signed int LL;
typedef long long unsigned int LU;

#define incID(i, l, r) for (int i = (l); i < (r); i++)
#define incII(i, l, r) for (int i = (l); i <= (r); i++)
#define decID(i, l, r) for (int i = (r)-1; i >= (l); i--)
#define decII(i, l, r) for (int i = (r); i >= (l); i--)
#define inc(i, n) incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define dec(i, n) decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)

#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define inID(v, l, r) ((l) <= (v) && (v) < (r))

#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define UB upper_bound
#define LB lower_bound
#define PQ priority_queue

#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
#define FOR(it, v) for (auto it = v.begin(); it != v.end(); ++it)
#define RFOR(it, v) for (auto it = v.rbegin(); it != v.rend(); ++it)

template <typename T> bool setmin(T &a, T b) {
  if (b < a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmax(T &a, T b) {
  if (b > a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmineq(T &a, T b) {
  if (b <= a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmaxeq(T &a, T b) {
  if (b >= a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

// ---- ----

int n;
string s;
int ans[100000];

int main() {
  cin >> n >> s;
  s[n + 0] = s[0];
  s[n + 1] = s[1];

  bool flag = false;
  inc(a, 4) {
    ans[0] = a / 2;
    ans[1] = a % 2;
    inc(i, n) {
      ans[i + 2] = (((ans[i + 1] + ans[i]) % 2 == 0) == (s[i + 1] == 'o'));
    }

    if (ans[0] == ans[n] && ans[1] == ans[n + 1]) {
      flag = true;
      break;
    }
  }

  if (flag) {
    inc(i, n) { cout << (ans[i] == 1 ? "S" : "W"); }
    cout << endl;
  } else {
    cout << -1 << endl;
  }

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

typedef long long signed int LL;
typedef long long unsigned int LU;

#define incID(i, l, r) for (int i = (l); i < (r); i++)
#define incII(i, l, r) for (int i = (l); i <= (r); i++)
#define decID(i, l, r) for (int i = (r)-1; i >= (l); i--)
#define decII(i, l, r) for (int i = (r); i >= (l); i--)
#define inc(i, n) incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define dec(i, n) decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)

#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define inID(v, l, r) ((l) <= (v) && (v) < (r))

#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define UB upper_bound
#define LB lower_bound
#define PQ priority_queue

#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
#define FOR(it, v) for (auto it = v.begin(); it != v.end(); ++it)
#define RFOR(it, v) for (auto it = v.rbegin(); it != v.rend(); ++it)

template <typename T> bool setmin(T &a, T b) {
  if (b < a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmax(T &a, T b) {
  if (b > a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmineq(T &a, T b) {
  if (b <= a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> bool setmaxeq(T &a, T b) {
  if (b >= a) {
    a = b;
    return true;
  } else {
    return false;
  }
}
template <typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

// ---- ----

int n;
string s;
int ans[100002];

int main() {
  cin >> n >> s;
  s[n + 0] = s[0];
  s[n + 1] = s[1];

  bool flag = false;
  inc(a, 4) {
    ans[0] = a / 2;
    ans[1] = a % 2;
    inc(i, n) {
      ans[i + 2] = (((ans[i + 1] + ans[i]) % 2 == 0) == (s[i + 1] == 'o'));
    }

    if (ans[0] == ans[n] && ans[1] == ans[n + 1]) {
      flag = true;
      break;
    }
  }

  if (flag) {
    inc(i, n) { cout << (ans[i] == 1 ? "S" : "W"); }
    cout << endl;
  } else {
    cout << -1 << endl;
  }

  return 0;
}