Permutation Sequence
<p>The set <code>[1, 2, 3, ...,&nbsp;n]</code> contains a total of <code>n!</code> unique permutations.</p>

<p>By listing and labeling all of the permutations in order, we get the following sequence for <code>n = 3</code>:</p>

<ol>
	<li><code>&quot;123&quot;</code></li>
	<li><code>&quot;132&quot;</code></li>
	<li><code>&quot;213&quot;</code></li>
	<li><code>&quot;231&quot;</code></li>
	<li><code>&quot;312&quot;</code></li>
	<li><code>&quot;321&quot;</code></li>
</ol>

<p>Given <code>n</code> and <code>k</code>, return the <code>k<sup>th</sup></code> permutation sequence.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> n = 3, k = 3
<strong>Output:</strong> "213"
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> n = 4, k = 9
<strong>Output:</strong> "2314"
</pre><p><strong class="example">Example 3:</strong></p>
<pre><strong>Input:</strong> n = 3, k = 1
<strong>Output:</strong> "123"
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 9</code></li>
	<li><code>1 &lt;= k &lt;= n!</code></li>
</ul>
Recursion; Math
public class Solution {
    public string GetPermutation(int n, int k) {
        var ans = new StringBuilder();
        int vis = 0;
        for (int i = 0; i < n; ++i) {
            int fact = 1;
            for (int j = 1; j < n - i; ++j) {
                fact *= j;
            }
            for (int j = 1; j <= n; ++j) {
                if (((vis >> j) & 1) == 0) {
                    if (k > fact) {
                        k -= fact;
                    } else {
                        ans.Append(j);
                        vis |= 1 << j;
                        break;
                    }
                }
            }
        }
        return ans.ToString();
    }
}
