1 bool dfs(vector<vector<char> > &board, int r, int c, string word) { 2   int m = board.size(); 3   int n = board[0].size(); 4   int dir[4][2] = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}}; 5  6   if (word.empty()) 7     return true; 8  9   if (r >= 0 && r < m && c >= 0 && c < n && board[r][c] == word[0]) {10     for (int i = 0; i < 4; i++) {11       char tmp = board[r][c];12       board[r][c] = 0;13       if (dfs(board, r + dir[i][0], c + dir[i][1], word.substr(1)))14         return true;15       board[r][c] = tmp;16     }17   }18 19   return false;20 }21 22 bool exist(vector<vector<char> > &board, string word) {23   if (board.empty() || board[0].empty()) return false;24 25   int m = board.size();26   int n = board[0].size();27 28   for (int i = 0; i < m; i++)29     for (int j = 0; j < n; j++)30       if (dfs(board, i, j, word))31         return true;32 33   return false;34 }