LeetCode Lexicographical Numbers

原题链接在这里：https://leetcode.com/problems/lexicographical-numbers/description/

题目：

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.

题解：

从cur=1开始加入res中，下个数有三种情况：

第一种, cur*10 <= n, 就把cur 乘以 10.

第二种, cur 末位不是9 并且 cur+1<=n的情况下, cur++.

第三种, 末位已经等于9, 或者cur现在增加到n, 此时把cur调整到前一位不为9的level.

Time Complexity: O(n). Space: O(1).

AC Java: 

class Solution {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> res = new ArrayList<Integer>();
        int cur = 1;
        for(int i = 0; i<n; i++){
            res.add(cur);
            if(cur*10 <= n){
                cur *= 10;
            }else if(cur%10!=9 && cur<n){
                cur++;
            }else{
                while((cur/10)%10 == 9){
                    cur /= 10;
                }
                cur = cur/10+1;
            }
        }
        return res;
    }
}