Leetcode - Strobogrammatic Numbe

My code:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Solution {
    int[] map = new int[10];
    public List<String> findStrobogrammatic(int n) {
        List<String> ret = new ArrayList<String>();
        if (n <= 0) {
            return ret;
        }
        Arrays.fill(map, -1);
        map[0] = 0;
        map[1] = 1;
        map[6] = 9;
        map[8] = 8;
        map[9] = 6;
        char[] board = new char[n];
        helper(0, board.length - 1, n, board, ret);
        
        return ret;
    }
    
    private void helper(int begin, int end, int n, char[] board, List<String> ret) {
        if (begin > end) {
            ret.add(String.valueOf(board));
            return;
        }
        else if (begin == end) {
            board[n / 2] = '0';
            helper(begin + 1, end - 1, n, board, ret);
            board[n / 2] = '1';
            helper(begin + 1, end - 1, n, board, ret);
            board[n / 2] = '8';
            helper(begin + 1, end - 1, n, board, ret);
        }
        else {
            for (int i = 0; i < 10; i++) {
                if ((begin == 0 && i == 0) || map[i] == -1) {
                    continue;
                }
                else {
                    board[begin] = (char) (i + '0');
                    board[end] = (char) (map[i] + '0');
                    helper(begin + 1, end - 1, n, board, ret);
                }
            }
        }
    }
    
    public static void main(String[] args) {
        Solution test = new Solution();
        List<String> ret = test.findStrobogrammatic(3);
        System.out.println(ret.toString());
    }
}

自己写了出来。
采用的 backtracking 的思想。
答案和这个差不多。

Anyway, Good luck, Richardo! -- 09/22/2016