Problem p03018 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>

#define ll long long
#define str string
#define pii pair<int, int>
#define pll pair<ll, ll>
#define fi first
#define se second

#define vc vector<char>
#define vvc vector<vc>
#define vi vector<int>
#define vll vector<ll>
#define vvi vector<vi>
#define vvll vector<vll>
#define vvvll vector<vvll>
#define vs vector<str>
#define vvs vector<vs>
#define vpii vector<pii>
#define vvpii vector<vpii>
#define vpll vector<pll>
#define vvpll vector<vpll>
#define vb vector<bool>
#define vvb vector<vb>
#define rep(i, a, b) for (int i = (a); i < int(b); i++)
#define repi(i, a, b) for (int i = (a); i <= int(b); i++)

using namespace std;
ll INF = LONG_LONG_MAX;
ll mod = 1000000000 + 7;

template <typename T, typename L>
void read(vector<T> &_data, L &_size, bool _shift) {
  _data.resize(_size + (ll)_shift);
  for (ll i = (ll)_shift; i < _size + (ll)_shift; i++)
    cin >> _data[i];
}

template <typename T, typename L>
void read(vector<vector<T>> &_data, L &_rows, L &_cols, bool _shiftRows,
          bool _shiftCols) {
  _data.resize(_rows + (ll)_shiftRows);
  for (ll i = 0; i < _rows + (ll)_shiftRows; i++)
    _data[i].resize(_cols + (ll)_shiftCols);
  for (ll i = (ll)_shiftRows; i < _rows + (ll)_shiftRows; i++)
    for (ll j = (ll)_shiftCols; j < _cols + (ll)_shiftCols; j++)
      cin >> _data[i][j];
}

template <typename T> void write(vector<T> &_data, bool _shift) {
  for (ll i = (ll)_shift; i < _data.size(); i++)
    cout << _data[i] << " ";
  cout << endl;
}

// TODO: SOLUTION

void solve() {
  str s;
  cin >> s;
  ll ans = 0;
  for (int i = s.size() - 3; i >= 0; i--) {
    for (int j = i; j < s.size() - 2; j++) {
      if (s.substr(j, 3) == "ABC") {
        ans++;
        s[j] = 'B';
        s[j + 1] = 'C';
        s[j + 2] = 'A';
      }
    }
  }
  cout << ans << endl;
}

int main() {

  // TODO: Set value of this variable manually
  bool _multipleTestCases = false;

  if (_multipleTestCases) {
    ll t;
    cin >> t;
    while (t--)
      solve();
  } else {
    solve();
  }

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>

#define ll long long
#define str string
#define pii pair<int, int>
#define pll pair<ll, ll>
#define fi first
#define se second

#define vc vector<char>
#define vvc vector<vc>
#define vi vector<int>
#define vll vector<ll>
#define vvi vector<vi>
#define vvll vector<vll>
#define vvvll vector<vvll>
#define vs vector<str>
#define vvs vector<vs>
#define vpii vector<pii>
#define vvpii vector<vpii>
#define vpll vector<pll>
#define vvpll vector<vpll>
#define vb vector<bool>
#define vvb vector<vb>
#define rep(i, a, b) for (int i = (a); i < int(b); i++)
#define repi(i, a, b) for (int i = (a); i <= int(b); i++)

using namespace std;
ll INF = LONG_LONG_MAX;
ll mod = 1000000000 + 7;

template <typename T, typename L>
void read(vector<T> &_data, L &_size, bool _shift) {
  _data.resize(_size + (ll)_shift);
  for (ll i = (ll)_shift; i < _size + (ll)_shift; i++)
    cin >> _data[i];
}

template <typename T, typename L>
void read(vector<vector<T>> &_data, L &_rows, L &_cols, bool _shiftRows,
          bool _shiftCols) {
  _data.resize(_rows + (ll)_shiftRows);
  for (ll i = 0; i < _rows + (ll)_shiftRows; i++)
    _data[i].resize(_cols + (ll)_shiftCols);
  for (ll i = (ll)_shiftRows; i < _rows + (ll)_shiftRows; i++)
    for (ll j = (ll)_shiftCols; j < _cols + (ll)_shiftCols; j++)
      cin >> _data[i][j];
}

template <typename T> void write(vector<T> &_data, bool _shift) {
  for (ll i = (ll)_shift; i < _data.size(); i++)
    cout << _data[i] << " ";
  cout << endl;
}

// TODO: SOLUTION

void solve() {
  str s;
  cin >> s;
  ll ans = 0;
  ll bcsuf = 0;
  for (int i = s.size() - 2; i >= 0; i--) {
    str sub;
    sub.resize(2);
    sub[0] = s[i];
    sub[1] = s[i + 1];
    if (sub == "BC")
      bcsuf++;
    else if (sub == "BA" || sub == "BB" || sub == "CC" || sub == "AC")
      bcsuf = 0;
    else if (sub == "AB" || sub == "AA")
      ans += bcsuf;
  }
  cout << ans << endl;
}

int main() {

  // TODO: Set value of this variable manually
  bool _multipleTestCases = false;

  if (_multipleTestCases) {
    ll t;
    cin >> t;
    while (t--)
      solve();
  } else {
    solve();
  }

  return 0;
}