class Solution {
    public boolean exist(char[][] board, String word) {
        if (board == null || board.length == 0 || board[0].length == 0) {
            return false;
        }
        int width = board.length;
        int height = board[0].length;
        boolean[][] used  = new boolean[width][height];
        for (int i=0; i<width; i++) {
            for (int j=0; j<height; j++) {
                if (search(i, j, board, used, word, 0)) {
                    return true;
                }
            }
        }
        return false;
    }

    public boolean search(int x, int y, char[][] board, boolean[][] used, String word, int index) {
        if (index >= word.length()) {
            return true;
        }
        if (x<0 || y<0 || x>=board.length || y >= board[0].length || board[x][y] != word.charAt(index) || used[x][y]) {
            return false;
        }
        used[x][y] = true;
        if (search(x-1, y, board, used, word, index+1) ||
            search(x, y-1, board, used, word, index+1) ||
            search(x, y+1, board, used, word, index+1) ||
            search(x+1, y, board, used, word, index+1)) {
            return true;
        }
        used[x][y] = false;
        return false;
    }
}