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.

分析：递归；

1

10 11-...-19

100-101-..-109  110-112-...-119

1000-1001-...-1009 1010-1011-..1019

先处理左边的，先处理上面的。一样的问题。

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