leetcode-problemset/leetcode/problem/number-of-unique-good-subsequences.html

<p>You are given a binary string <code>binary</code>. A <strong>subsequence</strong> of <code>binary</code> is considered <strong>good</strong> if it is <strong>not empty</strong> and has <strong>no leading zeros</strong> (with the exception of <code>&quot;0&quot;</code>).</p>

<p>Find the number of <strong>unique good subsequences</strong> of <code>binary</code>.</p>

<ul>
	<li>For example, if <code>binary = &quot;001&quot;</code>, then all the <strong>good</strong> subsequences are <code>[&quot;0&quot;, &quot;0&quot;, &quot;1&quot;]</code>, so the <strong>unique</strong> good subsequences are <code>&quot;0&quot;</code> and <code>&quot;1&quot;</code>. Note that subsequences <code>&quot;00&quot;</code>, <code>&quot;01&quot;</code>, and <code>&quot;001&quot;</code> are not good because they have leading zeros.</li>
</ul>

<p>Return <em>the number of <strong>unique good subsequences</strong> of </em><code>binary</code>. Since the answer may be very large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>

<p>A <strong>subsequence</strong> is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements.</p>

<p>&nbsp;</p>
<p><strong>Example 1:</strong></p>

<pre>
<strong>Input:</strong> binary = &quot;001&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> The good subsequences of binary are [&quot;0&quot;, &quot;0&quot;, &quot;1&quot;].
The unique good subsequences are &quot;0&quot; and &quot;1&quot;.
</pre>

<p><strong>Example 2:</strong></p>

<pre>
<strong>Input:</strong> binary = &quot;11&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> The good subsequences of binary are [&quot;1&quot;, &quot;1&quot;, &quot;11&quot;].
The unique good subsequences are &quot;1&quot; and &quot;11&quot;.</pre>

<p><strong>Example 3:</strong></p>

<pre>
<strong>Input:</strong> binary = &quot;101&quot;
<strong>Output:</strong> 5
<strong>Explanation:</strong> The good subsequences of binary are [&quot;1&quot;, &quot;0&quot;, &quot;1&quot;, &quot;10&quot;, &quot;11&quot;, &quot;101&quot;]. 
The unique good subsequences are &quot;0&quot;, &quot;1&quot;, &quot;10&quot;, &quot;11&quot;, and &quot;101&quot;.
</pre>

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

<ul>
	<li><code>1 &lt;= binary.length &lt;= 10<sup>5</sup></code></li>
	<li><code>binary</code> consists of only <code>&#39;0&#39;</code>s and <code>&#39;1&#39;</code>s.</li>
</ul>