Problem p01646 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <iostream>
#include <queue>
#include <string>
#include <vector>

using namespace std;

class Graph {
  typedef vector<int> Vector;
  typedef vector<Vector> Matrix;
  typedef queue<int> IntQueue;

public:
  Graph(int size) : size_(size), next_(size) {}
  void add_edge(int a, int b) { next_[a].push_back(b); }
  bool exists_path(int a, int b) {
    vector<bool> has_visited(size_, false);
    IntQueue q;
    q.push(a);
    has_visited[a] = true;
    while (not q.empty()) {
      int x = q.front();
      q.pop();
      for (size_t i = 0; i < next_[x].size(); ++i) {
        int y = next_[x][i];
        if (not has_visited[y]) {
          if (y == b) {
            return true;
          }
          has_visited[y] = true;
          q.push(y);
        }
      }
    }
    return false;
  }

private:
  int size_;
  Matrix next_;
};

bool solve(int n, string s[]) {
  Graph g(26);
  for (int i = 0; i < n - 1; ++i) {
    size_t pos = 0;
    while (pos < s[i].length() and s[i][pos] == s[i + 1][pos]) {
      ++pos;
    }
    if (pos == s[i].length()) {
      continue;
    } else if (pos == s[i + 1].length()) {
      return false;
    }
    g.add_edge(s[i][pos] - 'a', s[i + 1][pos] - 'a');
    for (int i = 0; i < 26; ++i) {
      for (int j = i + 1; j < 26; ++j) {
        if (g.exists_path(i, j) and g.exists_path(j, i)) {
          return false;
        }
      }
    }
  }
  return true;
}

int main() {
  int n;
  string s[500];
  while (true) {
    cin >> n;
    if (n == 0) {
      return 0;
    }
    for (int i = 0; i < n; ++i) {
      cin >> s[i];
    }
    if (solve(n, s)) {
      cout << "yes" << endl;
    } else {
      cout << "no" << endl;
    }
  }
}

Error: TLE

Fixed submission:

#include <iostream>
#include <queue>
#include <string>
#include <vector>

using namespace std;

class Graph {
  typedef vector<int> Vector;
  typedef vector<Vector> Matrix;
  typedef queue<int> IntQueue;

public:
  Graph(int size) : size_(size), next_(size) {}
  void add_edge(int a, int b) {
    if (0 <= a and a < size_ and 0 <= b and b < size_) {
      next_[a].push_back(b);
    } else {
      while (true) {
        a += 1;
      }
    }
  }
  bool exists_path(int a, int b) {
    vector<bool> has_visited(size_, false);
    IntQueue q;
    q.push(a);
    has_visited[a] = true;
    while (not q.empty()) {
      int x = q.front();
      q.pop();
      for (size_t i = 0; i < next_[x].size(); ++i) {
        int y = next_[x][i];
        if (not has_visited[y]) {
          if (y == b) {
            return true;
          }
          has_visited[y] = true;
          q.push(y);
        }
      }
    }
    return false;
  }

private:
  int size_;
  Matrix next_;
};

bool solve(int n, string s[]) {
  Graph g(26);
  for (int i = 0; i < n - 1; ++i) {
    size_t pos = 0;
    while (s[i][pos] == s[i + 1][pos]) {
      ++pos;
    }
    if (pos == s[i].length()) {
      continue;
    } else if (pos == s[i + 1].length()) {
      return false;
    }
    g.add_edge(s[i][pos] - 'a', s[i + 1][pos] - 'a');
    for (int i = 0; i < 26; ++i) {
      for (int j = i + 1; j < 26; ++j) {
        if (g.exists_path(i, j) and g.exists_path(j, i)) {
          return false;
        }
      }
    }
  }
  return true;
}

int main() {
  int n;
  string s[500];
  while (true) {
    cin >> n;
    if (n == 0) {
      return 0;
    }
    for (int i = 0; i < n; ++i) {
      cin >> s[i];
    }
    if (solve(n, s)) {
      cout << "yes" << endl;
    } else {
      cout << "no" << endl;
    }
  }
}