Problem p01811 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> P;
typedef vector<ll> V;
typedef complex<double> Point;

#define PI acos(-1.0)
#define EPS 1e-10
const ll INF = 1e16;
// const ll MOD = 1e9 + 7;
const ll MOD = 998244353;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define rep(i, N) for (int i = 0; i < (N); i++)
#define ALL(s) (s).begin(), (s).end()
#define EQ(a, b) (abs((a) - (b)) < EPS)
#define EQV(a, b) (EQ((a).real(), (b).real()) && EQ((a).imag(), (b).imag()))
#define fi first
#define se second
#define N_SIZE (1LL << 20)
#define NIL -1

ll sq(ll num) { return num * num; }
ll mod_pow(ll x, ll n) {
  if (n == 0)
    return 1;
  if (n == 1)
    return x % MOD;
  ll res = sq(mod_pow(x, n / 2));
  res %= MOD;
  if (n % 2 == 1) {
    res *= x;
    res %= MOD;
  }
  return res;
}
ll mod_add(ll a, ll b) { return (a + b) % MOD; }
ll mod_sub(ll a, ll b) { return (a - b + MOD) % MOD; }
ll mod_mul(ll a, ll b) { return a * b % MOD; }

string s;

int main() {
  cin >> s;
  bool f;
  while (s.size() > 3) {
    f = true;
    string t = "";
    char c = 'B';
    if (s.substr(0, 3) == "ABC")
      c = 'A';
    else if (s.substr(s.size() - 3, 3) == "ABC")
      c = 'C';
    rep(i, s.size()) {
      if (s.substr(i, 3) == "ABC") {
        i += 2;
        t += c;
      } else {
        t += s[i];
        if (s[i] == c)
          f = false;
      }
    }
    if (!f || s.size() == t.size())
      break;
    s = t;
  }
  if (s == "ABC")
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}


Error: TLE

Fixed submission:

#include <algorithm>
#include <bitset>
#include <cmath>
#include <complex>
#include <cstdio>
#include <fstream>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <tuple>
#include <utility>
#include <vector>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;
typedef vector<ll> V;
typedef complex<double> Point;

#define PI acos(-1.0)
#define EPS 1e-10
const ll INF = (1LL << 60) - 1;
const ll MOD = 1e9 + 7;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define rep(i, N) for (int i = 0; i < (N); i++)
#define ALL(s) (s).begin(), (s).end()
#define EQ(a, b) (abs((a) - (b)) < EPS)
#define EQV(a, b) (EQ((a).real(), (b).real()) && EQ((a).imag(), (b).imag()))
#define fi first
#define se second
#define N_SIZE (1LL << 20)
#define NIL -1
#define MAX_N 100100 * 3

string t, tt = "ABC";

bool solve(string str) {
  if (str == "ABC")
    return true;
  if (str.size() <= 3)
    return false;
  bool res = false;
  rep(i, 3) {
    string strr = "A";
    strr[0] += i;
    string buf = "";
    bool f = 0;
    rep(j, str.size()) {
      string b;
      if (j < str.size() - 2)
        b = str.substr(j, 3);
      if (b == tt) {
        buf += strr;
        j += 2;
        f = 1;
      } else
        buf += str[j];
    }
    if (f)
      res |= solve(buf);
  }
  return res;
}

int main() {
  cin >> t;
  if (solve(t))
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}