LC Note

public boolean exist(char[][] board, String word) {
        // write your code here
        if (board == null || board.length == 0) {
        return false; 
        }

        int m = board.length;
        int n = board[0].length;
        boolean[][] visit = new boolean[m][n];
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < board[0].length; j++) {
                    if (dfs(board, word, i, j, 0, visit))
                    return true;
            }
        }
        return false;
    }
    public boolean dfs(char[][] board, String word, int i, int j, int index, boolean[][] visit) {
        if (index == word.length())
        return true;

        int m = board.length;
        int n = board[0].length;
        if (i < 0 || i >= m || j < 0 || j>= n || board[i][j] != word.charAt(index)|| visit[i][j])
        return false;

        visit[i][j] = true;
       if ( dfs(board, word, i + 1, j, index + 1, visit) ||
            dfs(board, word, i - 1, j, index + 1, visit) ||
            dfs(board, word, i, j + 1, index + 1, visit) ||
            dfs(board, word, i, j - 1, index + 1, visit)
          ){
        return true;
          }   
        visit[i][j] = false;
        return false;
    }