count-palindromic-subsequences

Count Palindromic Subsequences

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countPalindromes

Given a string of digits <code>s</code>, return the number of palindromic subsequences of <code>s</code> having length <code>5</code>. Since the answer may be very large, return it modulo <code>109 + 7</code>.

Note:

<ul>
	<li>A string is palindromic if it reads the same forward and backward.</li>
	<li>A subsequence is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.</li>
</ul>

&nbsp;
Example 1:

<pre>
Input: s = &quot;103301&quot;
Output: 2
Explanation: 
There are 6 possible subsequences of length 5: &quot;10330&quot;,&quot;10331&quot;,&quot;10301&quot;,&quot;10301&quot;,&quot;13301&quot;,&quot;03301&quot;. 
Two of them (both equal to &quot;10301&quot;) are palindromic.
</pre>

Example 2:

<pre>
Input: s = &quot;0000000&quot;
Output: 21
Explanation: All 21 subsequences are &quot;00000&quot;, which is palindromic.
</pre>

Example 3:

<pre>
Input: s = &quot;9999900000&quot;
Output: 2
Explanation: The only two palindromic subsequences are &quot;99999&quot; and &quot;00000&quot;.
</pre>

&nbsp;
Constraints:

<ul>
	<li><code>1 &lt;= s.length &lt;= 104</code></li>
	<li><code>s</code> consists of digits.</li>
</ul>

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