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52 lines
2.0 KiB
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<p>Given a string <code>s</code>, return <em>the number of <strong>unique palindromes of length three</strong> that are a <strong>subsequence</strong> of </em><code>s</code>.</p>
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<p>Note that even if there are multiple ways to obtain the same subsequence, it is still only counted <strong>once</strong>.</p>
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<p>A <strong>palindrome</strong> is a string that reads the same forwards and backwards.</p>
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<p>A <strong>subsequence</strong> of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.</p>
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<ul>
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<li>For example, <code>"ace"</code> is a subsequence of <code>"<u>a</u>b<u>c</u>d<u>e</u>"</code>.</li>
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</ul>
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<p> </p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> s = "aabca"
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<strong>Output:</strong> 3
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<strong>Explanation:</strong> The 3 palindromic subsequences of length 3 are:
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- "aba" (subsequence of "<u>a</u>a<u>b</u>c<u>a</u>")
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- "aaa" (subsequence of "<u>aa</u>bc<u>a</u>")
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- "aca" (subsequence of "<u>a</u>ab<u>ca</u>")
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> s = "adc"
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<strong>Output:</strong> 0
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<strong>Explanation:</strong> There are no palindromic subsequences of length 3 in "adc".
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> s = "bbcbaba"
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<strong>Output:</strong> 4
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<strong>Explanation:</strong> The 4 palindromic subsequences of length 3 are:
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- "bbb" (subsequence of "<u>bb</u>c<u>b</u>aba")
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- "bcb" (subsequence of "<u>b</u>b<u>cb</u>aba")
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- "bab" (subsequence of "<u>b</u>bcb<u>ab</u>a")
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- "aba" (subsequence of "bbcb<u>aba</u>")
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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>3 <= s.length <= 10<sup>5</sup></code></li>
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<li><code>s</code> consists of only lowercase English letters.</li>
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</ul>
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