leetcode-problemset/leetcode/problem/find-kth-bit-in-nth-binary-string.html

<p>Given two positive integers <code>n</code> and <code>k</code>, the binary string <code>S<sub>n</sub></code> is formed as follows:</p>

<ul>
	<li><code>S<sub>1</sub> = &quot;0&quot;</code></li>
	<li><code>S<sub>i</sub> = S<sub>i - 1</sub> + &quot;1&quot; + reverse(invert(S<sub>i - 1</sub>))</code> for <code>i &gt; 1</code></li>
</ul>

<p>Where <code>+</code> denotes the concatenation operation, <code>reverse(x)</code> returns the reversed string <code>x</code>, and <code>invert(x)</code> inverts all the bits in <code>x</code> (<code>0</code> changes to <code>1</code> and <code>1</code> changes to <code>0</code>).</p>

<p>For example, the first four strings in the above sequence are:</p>

<ul>
	<li><code>S<sub>1 </sub>= &quot;0&quot;</code></li>
	<li><code>S<sub>2 </sub>= &quot;0<strong>1</strong>1&quot;</code></li>
	<li><code>S<sub>3 </sub>= &quot;011<strong>1</strong>001&quot;</code></li>
	<li><code>S<sub>4</sub> = &quot;0111001<strong>1</strong>0110001&quot;</code></li>
</ul>

<p>Return <em>the</em> <code>k<sup>th</sup></code> <em>bit</em> <em>in</em> <code>S<sub>n</sub></code>. It is guaranteed that <code>k</code> is valid for the given <code>n</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 3, k = 1
<strong>Output:</strong> &quot;0&quot;
<strong>Explanation:</strong> S<sub>3</sub> is &quot;<strong><u>0</u></strong>111001&quot;.
The 1<sup>st</sup> bit is &quot;0&quot;.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 4, k = 11
<strong>Output:</strong> &quot;1&quot;
<strong>Explanation:</strong> S<sub>4</sub> is &quot;0111001101<strong><u>1</u></strong>0001&quot;.
The 11<sup>th</sup> bit is &quot;1&quot;.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 20</code></li>
	<li><code>1 &lt;= k &lt;= 2<sup>n</sup> - 1</code></li>
</ul>