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44 lines
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44 lines
1.7 KiB
HTML
<p>Given a string of digits <code>s</code>, return <em>the number of <strong>palindromic subsequences</strong> of</em> <code>s</code><em> having length </em><code>5</code>. Since the answer may be very large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>
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<p><strong>Note:</strong></p>
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<ul>
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<li>A string is <strong>palindromic</strong> if it reads the same forward and backward.</li>
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<li>A <strong>subsequence</strong> 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>
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</ul>
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<p> </p>
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<p><strong class="example">Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> s = "103301"
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<strong>Output:</strong> 2
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<strong>Explanation:</strong>
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There are 6 possible subsequences of length 5: "10330","10331","10301","10301","13301","03301".
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Two of them (both equal to "10301") are palindromic.
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</pre>
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<p><strong class="example">Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> s = "0000000"
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<strong>Output:</strong> 21
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<strong>Explanation:</strong> All 21 subsequences are "00000", which is palindromic.
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</pre>
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<p><strong class="example">Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> s = "9999900000"
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<strong>Output:</strong> 2
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<strong>Explanation:</strong> The only two palindromic subsequences are "99999" and "00000".
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</pre>
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<p> </p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 <= s.length <= 10<sup>4</sup></code></li>
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<li><code>s</code> consists of digits.</li>
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</ul>
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