Problem p03565 (C++)

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

using ll = long long;

template <typename T> T gcd(T a, T b) {
  if (a < b)
    return gcd(b, a);
  T r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

template <typename T> T lcm(T a, T b) { return a * b / gcd(a, b); }

template <typename T> T ext_gcd(T a, T b, T &x, T &y) {
  x = 1;
  y = 0;
  T x0 = 0, y0 = 1;
  while (b != 0) {
    T q = a / b;
    a -= q * b;
    swap(a, b);
    x -= q * x0;
    swap(x, x0);
    y -= q * y0;
    swap(y, y0);
  }
  return a;
}

template <typename T> T inv_mod(T a, T mod) {
  T t1, t2;
  ext_gcd(a, mod, t1, t2);
  t1 %= mod;
  if (t1 < 0)
    t1 += mod;
  return t1;
}

template <typename T> T combination_large(T n, T k, T mod) {
  if (n < k || n < 0 || k < 0)
    return 0;
  vector<T> inv(n + 1, 1), fact(n + 1, 1), fact_inv(n + 1, 1);
  for (T i = 2; i <= n; ++i) {
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    fact[i] = (fact[i - 1] * i) % mod;
    fact_inv[i] = fact_inv[i - 1] * inv[i] % mod;
  }
  return fact[n] * (fact_inv[k] * fact_inv[n - k] % mod) % mod;
}

template <typename T> T combination(T n, T k, T mod = 1) {
  vector<vector<T>> table(n + 1, vector<T>(k + 1, 1));
  for (T i = 1; i <= n; ++i) {
    table[i][0] = 1;
    for (T j = 1; j <= n; ++j) {
      table[i][j] = (table[i - 1][j - 1] + table[i - 1][j]) % mod;
    }
  }
}

int main(int, char **) {
  string S_, T;
  cin >> S_ >> T;

  vector<string> candidate;
  for (size_t i = 0; i <= S_.size() - T.size(); ++i) {
    bool ok = true;
    for (size_t j = 0; j < T.size() && i + j < S_.size() && ok; ++j) {
      if (S_[i + j] == '?')
        continue;
      ok = S_[i + j] == T[j];
    }
    if (ok) {
      string S = S_.substr(0, i) + T;
      if (i + T.size() < S_.size()) {
        S += S_.substr(i + T.size());
      }
      replace(S.begin(), S.end(), '?', 'a');
      candidate.push_back(S);
    }
  }
  if (candidate.empty()) {
    cout << "UNRESTORABLE" << endl;
    return EXIT_SUCCESS;
  }
  cout << *min_element(candidate.begin(), candidate.end()) << endl;
  return 0;
}


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

using ll = long long;

template <typename T> T gcd(T a, T b) {
  if (a < b)
    return gcd(b, a);
  T r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

template <typename T> T lcm(T a, T b) { return a * b / gcd(a, b); }

template <typename T> T ext_gcd(T a, T b, T &x, T &y) {
  x = 1;
  y = 0;
  T x0 = 0, y0 = 1;
  while (b != 0) {
    T q = a / b;
    a -= q * b;
    swap(a, b);
    x -= q * x0;
    swap(x, x0);
    y -= q * y0;
    swap(y, y0);
  }
  return a;
}

template <typename T> T inv_mod(T a, T mod) {
  T t1, t2;
  ext_gcd(a, mod, t1, t2);
  t1 %= mod;
  if (t1 < 0)
    t1 += mod;
  return t1;
}

template <typename T> T combination_large(T n, T k, T mod) {
  if (n < k || n < 0 || k < 0)
    return 0;
  vector<T> inv(n + 1, 1), fact(n + 1, 1), fact_inv(n + 1, 1);
  for (T i = 2; i <= n; ++i) {
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    fact[i] = (fact[i - 1] * i) % mod;
    fact_inv[i] = fact_inv[i - 1] * inv[i] % mod;
  }
  return fact[n] * (fact_inv[k] * fact_inv[n - k] % mod) % mod;
}

template <typename T> T combination(T n, T k, T mod = 1) {
  vector<vector<T>> table(n + 1, vector<T>(k + 1, 1));
  for (T i = 1; i <= n; ++i) {
    table[i][0] = 1;
    for (T j = 1; j <= n; ++j) {
      table[i][j] = (table[i - 1][j - 1] + table[i - 1][j]) % mod;
    }
  }
}

int main(int, char **) {
  string S_, T;
  cin >> S_ >> T;

  vector<string> candidate;
  if (S_.size() < T.size()) {
    cout << "UNRESTORABLE" << endl;
    return EXIT_SUCCESS;
  }
  for (size_t i = 0; i <= S_.size() - T.size(); ++i) {
    bool ok = true;
    for (size_t j = 0; j < T.size() && i + j < S_.size() && ok; ++j) {
      if (S_[i + j] == '?')
        continue;
      ok = S_[i + j] == T[j];
    }
    if (ok) {
      string S = S_.substr(0, i) + T;
      if (i + T.size() < S_.size()) {
        S += S_.substr(i + T.size());
      }
      replace(S.begin(), S.end(), '?', 'a');
      candidate.push_back(S);
    }
  }
  if (candidate.empty()) {
    cout << "UNRESTORABLE" << endl;
    return EXIT_SUCCESS;
  }
  cout << *min_element(candidate.begin(), candidate.end()) << endl;
  return 0;
}