{ "data": { "question": { "questionId": "2059", "questionFrontendId": "1930", "boundTopicId": null, "title": "Unique Length-3 Palindromic Subsequences", "titleSlug": "unique-length-3-palindromic-subsequences", "content": "

Given a string s, return the number of unique palindromes of length three that are a subsequence of s.

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Note that even if there are multiple ways to obtain the same subsequence, it is still only counted once.

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A palindrome is a string that reads the same forwards and backwards.

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A subsequence 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.

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Example 1:

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\nInput: s = "aabca"\nOutput: 3\nExplanation: The 3 palindromic subsequences of length 3 are:\n- "aba" (subsequence of "aabca")\n- "aaa" (subsequence of "aabca")\n- "aca" (subsequence of "aabca")\n
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Example 2:

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\nInput: s = "adc"\nOutput: 0\nExplanation: There are no palindromic subsequences of length 3 in "adc".\n
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Example 3:

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\nInput: s = "bbcbaba"\nOutput: 4\nExplanation: The 4 palindromic subsequences of length 3 are:\n- "bbb" (subsequence of "bbcbaba")\n- "bcb" (subsequence of "bbcbaba")\n- "bab" (subsequence of "bbcbaba")\n- "aba" (subsequence of "bbcbaba")\n
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Constraints:

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