Convert to Base -2
<p>Given an integer <code>n</code>, return <em>a binary string representing its representation in base</em> <code>-2</code>.</p>

<p><strong>Note</strong> that the returned string should not have leading zeros unless the string is <code>&quot;0&quot;</code>.</p>

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

<pre>
<strong>Input:</strong> n = 2
<strong>Output:</strong> &quot;110&quot;
<strong>Explantion:</strong> (-2)<sup>2</sup> + (-2)<sup>1</sup> = 2
</pre>

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

<pre>
<strong>Input:</strong> n = 3
<strong>Output:</strong> &quot;111&quot;
<strong>Explantion:</strong> (-2)<sup>2</sup> + (-2)<sup>1</sup> + (-2)<sup>0</sup> = 3
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> n = 4
<strong>Output:</strong> &quot;100&quot;
<strong>Explantion:</strong> (-2)<sup>2</sup> = 4
</pre>

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

<ul>
	<li><code>0 &lt;= n &lt;= 10<sup>9</sup></code></li>
</ul>
Math
public class Solution {
    public string BaseNeg2(int n) {
        if (n == 0) {
            return "0";
        }
        int k = 1;
        StringBuilder ans = new StringBuilder();
        int num = n;
        while (num != 0) {
            if (num % 2 != 0) {
                ans.Append('1');
                num -= k;
            } else {
                ans.Append('0');
            }
            k *= -1;
            num /= 2;
        }
        char[] cs = ans.ToString().ToCharArray();
        Array.Reverse(cs);
        return new string(cs);
    }
}