Lexicographical Numbers
<p>Given an integer <code>n</code>, return all the numbers in the range <code>[1, n]</code> sorted in lexicographical order.</p>

<p>You must write an algorithm that runs in&nbsp;<code>O(n)</code>&nbsp;time and uses <code>O(1)</code> extra space.&nbsp;</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> n = 13
<strong>Output:</strong> [1,10,11,12,13,2,3,4,5,6,7,8,9]
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> n = 2
<strong>Output:</strong> [1,2]
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 5 * 10<sup>4</sup></code></li>
</ul>
Depth-First Search; Trie
class Solution {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> ans = new ArrayList<>(n);
        int v = 1;
        for (int i = 0; i < n; ++i) {
            ans.add(v);
            if (v * 10 <= n) {
                v *= 10;
            } else {
                while (v % 10 == 9 || v + 1 > n) {
                    v /= 10;
                }
                ++v;
            }
        }
        return ans;
    }
}