247. Strobogrammatic Number II

247. Strobogrammatic Number II

题目链接：https://leetcode.com/problems...

这题recursion和iteration都可以做，一种思路就是从中间开始往两边延伸，每次c[i-k], c[i+k]有5种可能性: (6, 9), (9, 6), (1, 1), (8, 8)和(0, 0)，其中开头处不能是0。可以加memo或者用dp table优化。

public class Solution {
    public List<String> findStrobogrammatic(int n) {
        List<String> dp = new ArrayList();
        if(n % 2 == 1) {
            dp.add("0"); dp.add("1"); dp.add("8");
        }
        else dp.add("");
        
        if(n <= 1) return dp;
        
        // get permutation of (0, n/2)
        String[] numbers = new String[] {"0", "1", "6", "8", "9"};
        String[] reverse = new String[] {"0", "1", "9", "8", "6"};
        for(int i = n / 2 - 1; i >= 0; i--) {
            List<String> temp = new ArrayList();
            for(String s : dp) {
                if(i != 0) temp.add(numbers[0] + s + reverse[0]);
                for(int j = 1; j < numbers.length; j++) {
                    temp.add(numbers[j] + s + reverse[j]);
                }
            }
            dp = temp;
        }
        
        return dp;
    }
}