386. Lexicographical Numbers

Given an integer n, return 1 - n in lexicographical order.

For example, given 13, return: [1,10,11,12,13,2,3,4,5,6,7,8,9].

Please optimize your algorithm to use less time and space. The input size may be as large as 5,000,000.

class Solution {
    private List<Integer> result;
    private int n;
    public List<Integer> lexicalOrder(int n) {
        this.result = new ArrayList<Integer>();
        this.n = n;
        solve(1);
        return result;
    }
    private void solve(int m) {
        result.add(m);
        if (m * 10 <= n) solve(m * 10);
        if (m < n && m % 10 < 9) solve(m + 1);
    }
}