Problem p03788 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double lf;
typedef pair<int, int> ii;
typedef pair<ii, int> iii;
#define REP(i, n) for (int i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define RST(i, n) memset(i, n, sizeof i)
#define SZ(a) (int)a.size()
#define ALL(a) a.begin(), a.end()
#define X first
#define Y second
#define eb emplace_back
#ifdef cold66
#define debug(...)                                                             \
  do {                                                                         \
    fprintf(stderr, "LINE %d: (%s) = ", __LINE__, #__VA_ARGS__);               \
    _do(__VA_ARGS__);                                                          \
  } while (0)
template <typename T> void _do(T &&_x) { cerr << _x << endl; }
template <typename T, typename... S> void _do(T &&_x, S &&..._t) {
  cerr << _x << ", ";
  _do(_t...);
}
template <typename _a, typename _b>
ostream &operator<<(ostream &_s, const pair<_a, _b> &_p) {
  return _s << "(" << _p.X << "," << _p.Y << ")";
}
template <typename It> ostream &_OUTC(ostream &_s, It _ita, It _itb) {
  _s << "{";
  for (It _it = _ita; _it != _itb; _it++) {
    _s << (_it == _ita ? "" : ",") << *_it;
  }
  _s << "}";
  return _s;
}
template <typename _a> ostream &operator<<(ostream &_s, vector<_a> &_c) {
  return _OUTC(_s, ALL(_c));
}
template <typename _a> ostream &operator<<(ostream &_s, set<_a> &_c) {
  return _OUTC(_s, ALL(_c));
}
template <typename _a, typename _b>
ostream &operator<<(ostream &_s, map<_a, _b> &_c) {
  return _OUTC(_s, ALL(_c));
}
template <typename _t> void pary(_t _a, _t _b) {
  _OUTC(cerr, _a, _b);
  cerr << endl;
}
#define IOS()
#else
#define debug(...)
#define pary(...)
#define endl '\n'
#define IOS()                                                                  \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);
#endif // cold66
//}
template <class T> inline bool chkmax(T &a, const T &b) {
  return b > a ? a = b, true : false;
}
template <class T> inline bool chkmin(T &a, const T &b) {
  return b < a ? a = b, true : false;
}

const ll MAXn = 1e5 + 5, MAXlg = __lg(MAXn) + 2;
const ll MOD = 1000000007;
const ll INF = 0x3f3f3f3f;

void pp(int x) {
  if (x == 0)
    cout << 'A';
  else
    cout << 'B';
}
int main() {
  IOS();
  ll n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  if (k > 2 * n)
    k = 2 * n + (k % 2);
  list<int> lst;
  REP(i, n) {
    if (s[i] == 'A')
      lst.push_back(0);
    else
      lst.push_back(1);
  }
  int flip = 0;
  debug(k);
  while (k--) {
    int cur = lst.front();
    lst.pop_front();
    if (flip)
      cur ^= 1;
    if (cur == 0) {
      if (flip)
        cur = 0;
      else
        cur = 1;
      lst.push_front(cur);
      continue;
    }
    if (flip)
      lst.push_back(0);
    else
      lst.push_back(1);
    for (auto i : lst)
      debug(k, i);
    flip ^= 1;
  }
  while (SZ(lst)) {
    int cur = lst.front();
    lst.pop_front();
    if (flip)
      pp(cur ^ 1);
    else
      pp(cur);
  }
  cout << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double lf;
typedef pair<int, int> ii;
typedef pair<ii, int> iii;
#define REP(i, n) for (int i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define RST(i, n) memset(i, n, sizeof i)
#define SZ(a) (int)a.size()
#define ALL(a) a.begin(), a.end()
#define X first
#define Y second
#define eb emplace_back
#ifdef cold66
#define debug(...)                                                             \
  do {                                                                         \
    fprintf(stderr, "LINE %d: (%s) = ", __LINE__, #__VA_ARGS__);               \
    _do(__VA_ARGS__);                                                          \
  } while (0)
template <typename T> void _do(T &&_x) { cerr << _x << endl; }
template <typename T, typename... S> void _do(T &&_x, S &&..._t) {
  cerr << _x << ", ";
  _do(_t...);
}
template <typename _a, typename _b>
ostream &operator<<(ostream &_s, const pair<_a, _b> &_p) {
  return _s << "(" << _p.X << "," << _p.Y << ")";
}
template <typename It> ostream &_OUTC(ostream &_s, It _ita, It _itb) {
  _s << "{";
  for (It _it = _ita; _it != _itb; _it++) {
    _s << (_it == _ita ? "" : ",") << *_it;
  }
  _s << "}";
  return _s;
}
template <typename _a> ostream &operator<<(ostream &_s, vector<_a> &_c) {
  return _OUTC(_s, ALL(_c));
}
template <typename _a> ostream &operator<<(ostream &_s, set<_a> &_c) {
  return _OUTC(_s, ALL(_c));
}
template <typename _a, typename _b>
ostream &operator<<(ostream &_s, map<_a, _b> &_c) {
  return _OUTC(_s, ALL(_c));
}
template <typename _t> void pary(_t _a, _t _b) {
  _OUTC(cerr, _a, _b);
  cerr << endl;
}
#define IOS()
#else
#define debug(...)
#define pary(...)
#define endl '\n'
#define IOS()                                                                  \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);
#endif // cold66
//}
template <class T> inline bool chkmax(T &a, const T &b) {
  return b > a ? a = b, true : false;
}
template <class T> inline bool chkmin(T &a, const T &b) {
  return b < a ? a = b, true : false;
}

const ll MAXn = 1e5 + 5, MAXlg = __lg(MAXn) + 2;
const ll MOD = 1000000007;
const ll INF = 0x3f3f3f3f;

void pp(int x) {
  if (x == 0)
    cout << 'A';
  else
    cout << 'B';
}
int main() {
  IOS();
  ll n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  if (k > 2 * n)
    k = 2 * n + (k % 2);
  list<int> lst;
  REP(i, n) {
    if (s[i] == 'A')
      lst.push_back(0);
    else
      lst.push_back(1);
  }
  int flip = 0;
  debug(k);
  while (k--) {
    int cur = lst.front();
    lst.pop_front();
    if (flip)
      cur ^= 1;
    if (cur == 0) {
      if (flip)
        cur = 0;
      else
        cur = 1;
      lst.push_front(cur);
      continue;
    }
    if (flip)
      lst.push_back(0);
    else
      lst.push_back(1);
    flip ^= 1;
  }
  while (SZ(lst)) {
    int cur = lst.front();
    lst.pop_front();
    if (flip)
      pp(cur ^ 1);
    else
      pp(cur);
  }
  cout << endl;
}