Leetcode#79 Word Search

依次枚举起始点，DFS+回溯
代码：
 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 }