Numbers With Same Consecutive Differences

class Solution {
    List<String> list = new ArrayList<>();
    public int[] numsSameConsecDiff(int N, int K) {
        if(N == 1) return new int[]{0,1,2,3,4,5,6,7,8,9};
        for(int i = 1; i<10; i++){
            addString(i+"", N, K, 1);
        }
        int[] res = new int[list.size()];
        for(int i = 0; i<list.size(); i++){
            res[i] = Integer.parseInt(list.get(i));
        }
        return res;
    }
    public void addString(String a, int N, int K, int ix){
        if(a.length() == N){
            list.add(a);
            return ;
        }
        for(int i = 0; i<10; i++){
            int prev = Integer.parseInt(a.substring(ix-1, ix));
            if(Math.abs(prev - i) == K){
                a += i+"";
                addString(a, N, K, ix+1);
                a = a.substring(0, a.length()-1);
            }
        }
    }
}