LeetCode 386. 字典序排数（DFS&循环）

例如，
给定 n =1 3，返回 [1,10,11,12,13,2,3,4,5,6,7,8,9] 。
请尽可能的优化算法的时间复杂度和空间复杂度。 输入的数据 n 小于等于 5,000,000。

class Solution {
public:
    vector<int> lexicalOrder(int n) {
    	vector<int> ans(n);	//n=1000
    	int cur = 1;
    	for(int i = 0; i < n; ++i)
    	{
    		ans[i] = cur;
    		if(cur*10 <= n)
    			cur *= 10;			//1，10,100,1000
    		else
    		{
    			if(cur >= n)		//1000
    				cur /= 10;
    			cur += 1;			//101。。。109
    			while(cur%10 == 0)	//110
    				cur /= 10;		//11
    		}
    	}
    	return ans;
    }
};

n = 1000
[1,10,100,1000,101,102,103,104,105,106,107,108,109,
11,110,111,112,113,114,115,116,117,118,119,12,120,
121,122,123,124,125,126,127,128,129,13,130,131,132,
133,134,135,136,137,138,139,14,140,141,142,143,144,
145,146,147,148,149,15,150,151,152,153,154,155,156,
157,158,159,16,160,161,162,163,164,165,166,167,168,.

class Solution {
	vector<int> ans;
public:
    vector<int> lexicalOrder(int n) {
        for(int i=1; i<=9; i++)
            dfs(i, n);
        return ans;
    }
    void dfs(int cur, int n)
    {
        if(cur > n)
        	return;
        ans.push_back(cur);
        for(int i=0;i<10;i++)
        {
            if(10*cur + i > n)
                return;
            dfs(10*cur+i, n);
        }
    }
};