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.

 佩服。。

public class Solution {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> res = new ArrayList<>();
        for(int i=1;i<10;++i){
          dfs(i, n, res); 
        }
        return res;
    }
    
    public void dfs(int cur, int n, List<Integer> res){
        if(cur>n)
            return;
        else{
            res.add(cur);
            for(int i=0;i<10;++i){
                if(10*cur+i>n)
                    return;
                dfs(10*cur+i, n, res);
            }
        }
    }
}

没想到是这样。。每个都看成一棵树，然后dfs