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<p>Write a function that takes an unsigned integer and returns the number of &#39;1&#39; bits it has (also known as the <a href="http://en.wikipedia.org/wiki/Hamming_weight" target="_blank">Hamming weight</a>).</p>
<p><strong>Note:</strong></p>
<ul>
<li>Note that in some languages, such as Java, there is no unsigned integer type. In this case, the input will be given as a signed integer type. It should not affect your implementation, as the integer&#39;s internal binary representation is the same, whether it is signed or unsigned.</li>
<li>In Java, the compiler represents the signed integers using <a href="https://en.wikipedia.org/wiki/Two%27s_complement" target="_blank">2&#39;s complement notation</a>. Therefore, in <strong>Example 3</strong>, the input represents the signed integer. <code>-3</code>.</li>
</ul>
<p>&nbsp;</p>
<p><strong>Example 1:</strong></p>
<pre>
<strong>Input:</strong> n = 00000000000000000000000000001011
<strong>Output:</strong> 3
<strong>Explanation:</strong> The input binary string <strong>00000000000000000000000000001011</strong> has a total of three &#39;1&#39; bits.
</pre>
<p><strong>Example 2:</strong></p>
<pre>
<strong>Input:</strong> n = 00000000000000000000000010000000
<strong>Output:</strong> 1
<strong>Explanation:</strong> The input binary string <strong>00000000000000000000000010000000</strong> has a total of one &#39;1&#39; bit.
</pre>
<p><strong>Example 3:</strong></p>
<pre>
<strong>Input:</strong> n = 11111111111111111111111111111101
<strong>Output:</strong> 31
<strong>Explanation:</strong> The input binary string <strong>11111111111111111111111111111101</strong> has a total of thirty one &#39;1&#39; bits.
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>
<ul>
<li>The input must be a <strong>binary string</strong> of length <code>32</code>.</li>
</ul>
<p>&nbsp;</p>
<strong>Follow up:</strong> If this function is called many times, how would you optimize it?