Problem p00017 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
using namespace std;
#define FOR(i, s, n) for (int i = s; i < (int)n; i++)
#define per(i, n) for (int i = n; i >= 0; i--)
#define ROF(i, s, n) for (int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\\n"
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define EACH(i, n)                                                             \\
  for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define CLR(a) memset((a), 0, sizeof(a))
#define NCLR(a) memset((a), -1, sizeof(a))
#define dump(x) cout << #x << " = " << (x) << "\\n";
#define debug(x)                                                               \\
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \\
       << " " << __FILE__ << "\\n";
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue<int, vector<int>, greater<int>> minpq;
typedef complex<double> P;
static const double EPS = 1e-10;
static const double PI = acos(-1.0);
static const int mod = 1000000007;
static const int INF = 1 << 30;
static const LL LL_INF = 1152921504606846976;
static const int dx[] = {-1, 0, 1, 0, 1, -1, 1, -1};
static const int dy[] = {0, -1, 0, 1, 1, 1, -1, -1};

string str;
map<char, char> change;

int main() {
  for (int i = 0; i < 26; i++) {
    change[i + 'a'] = (i + 25) % 26 + 'a';
  }
  while (getline(cin, str)) {
    for (;;) {
      for (int j = 0; j < str.size(); j++) {
        if (str[j] >= 'a' && str[j] <= 'z') {
          str[j] = change[str[j]];
        }
      }
      bool is_can = false;
      for (int j = 2; j < str.size(); j++) {
        if (str.substr(j - 2, 3) == "the") {
          is_can = true;
          break;
        }
      }
      for (int j = 3; j < str.size(); j++) {
        if (str.substr(j - 3, 4) == "that" || str.substr(j - 3, 4) == "this") {
          is_can = true;
          break;
        }
      }
      if (is_can) {
        break;
      }
    }
    printf("%s\\n", str.c_str());
  }
  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
#define FOR(i, s, n) for (int i = s; i < (int)n; i++)
#define per(i, n) for (int i = n; i >= 0; i--)
#define ROF(i, s, n) for (int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\\n"
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define EACH(i, n)                                                             \\
  for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define CLR(a) memset((a), 0, sizeof(a))
#define NCLR(a) memset((a), -1, sizeof(a))
#define dump(x) cout << #x << " = " << (x) << "\\n";
#define debug(x)                                                               \\
  cout << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \\
       << " " << __FILE__ << "\\n";
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue<int, vector<int>, greater<int>> minpq;
typedef complex<double> P;
static const double EPS = 1e-10;
static const double PI = acos(-1.0);
static const int mod = 1000000007;
static const int INF = 1 << 30;
static const LL LL_INF = 1152921504606846976;
static const int dx[] = {-1, 0, 1, 0, 1, -1, 1, -1};
static const int dy[] = {0, -1, 0, 1, 1, 1, -1, -1};

string str;

int main() {
  string str_copy;
  set<string> s;
  getline(cin, str);
  for (int i = 1;; i++) {
    str_copy = str;
    bool can_continue = false;
    for (int j = 0; j < str_copy.size(); j++) {
      if (str_copy[j] >= 'a' && str_copy[j] <= 'z') {
        str_copy[j] += 26;
        str_copy[j] -= i;
        str_copy[j] = ((str_copy[j] - 'a') % 26) + 'a';
        if (str_copy[j] < 'a' || str_copy[j] > 'z') {
          can_continue = true;
          break;
        }
      }
    }
    if (s.count(str_copy)) {
      continue;
    }
    s.insert(str_copy);
    if (can_continue) {
      continue;
    }
    bool is_can = false;
    for (int j = 0; j < str_copy.size() - 2; j++) {
      if (str_copy.substr(j, 3) == "the") {
        is_can = true;
        break;
      }
    }
    for (int j = 0; j < str_copy.size() - 3; j++) {
      if (str_copy.substr(j, 4) == "that" || str_copy.substr(j, 4) == "this") {
        is_can = true;
        break;
      }
    }
    if (is_can) {
      break;
    }
  }
  printf("%s\\n", str_copy.c_str());
  return 0;
}