Method lexicalOrder

LexicographicalNumbers.java:3–10

(int n)

//DFS
class Solution {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> res = new ArrayList<>();
        for(int i=1;i<10;i++){
            if(i>n) break;
            dfs(i,n,res);
        }
        return res;
    }
    public void dfs(int curNum, int target, List<Integer>res){
        if(curNum>target){
            return;