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leetcode/problem/distinct-subsequences-ii.html

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+<p>Given a string s, return <em>the number of <strong>distinct non-empty subsequences</strong> of</em> <code>s</code>. Since the answer may be very large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>
+A <strong>subsequence</strong> of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., <code>&quot;ace&quot;</code> is a subsequence of <code>&quot;<u>a</u>b<u>c</u>d<u>e</u>&quot;</code> while <code>&quot;aec&quot;</code> is not.
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;abc&quot;
+<strong>Output:</strong> 7
+<strong>Explanation:</strong> The 7 distinct subsequences are &quot;a&quot;, &quot;b&quot;, &quot;c&quot;, &quot;ab&quot;, &quot;ac&quot;, &quot;bc&quot;, and &quot;abc&quot;.
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;aba&quot;
+<strong>Output:</strong> 6
+<strong>Explanation:</strong> The 6 distinct subsequences are &quot;a&quot;, &quot;b&quot;, &quot;ab&quot;, &quot;aa&quot;, &quot;ba&quot;, and &quot;aba&quot;.
+</pre>
+
+<p><strong>Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;aaa&quot;
+<strong>Output:</strong> 3
+<strong>Explanation:</strong> The 3 distinct subsequences are &quot;a&quot;, &quot;aa&quot; and &quot;aaa&quot;.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= s.length &lt;= 2000</code></li>
+	<li><code>s</code> consists of lowercase English letters.</li>
+</ul>