{ "data": { "question": { "questionId": "730", "questionFrontendId": "730", "categoryTitle": "Algorithms", "boundTopicId": 1774, "title": "Count Different Palindromic Subsequences", "titleSlug": "count-different-palindromic-subsequences", "content": "

Given a string s, return the number of different non-empty palindromic subsequences in s. Since the answer may be very large, return it modulo 109 + 7.

\n\n

A subsequence of a string is obtained by deleting zero or more characters from the string.

\n\n

A sequence is palindromic if it is equal to the sequence reversed.

\n\n

Two sequences a1, a2, ... and b1, b2, ... are different if there is some i for which ai != bi.

\n\n

 

\n

Example 1:

\n\n
\nInput: s = "bccb"\nOutput: 6\nExplanation: The 6 different non-empty palindromic subsequences are 'b', 'c', 'bb', 'cc', 'bcb', 'bccb'.\nNote that 'bcb' is counted only once, even though it occurs twice.\n
\n\n

Example 2:

\n\n
\nInput: s = "abcdabcdabcdabcdabcdabcdabcdabcddcbadcbadcbadcbadcbadcbadcbadcba"\nOutput: 104860361\nExplanation: There are 3104860382 different non-empty palindromic subsequences, which is 104860361 modulo 109 + 7.\n
\n\n

 

\n

Constraints:

\n\n\n", "translatedTitle": "统计不同回文子序列", "translatedContent": "

给定一个字符串 s，返回 s 中不同的非空「回文子序列」个数 。

\n\n

通过从 s 中删除 0 个或多个字符来获得子序列。

\n\n

如果一个字符序列与它反转后的字符序列一致，那么它是「回文字符序列」。

\n\n

如果有某个 i , 满足 ai != bi ，则两个序列 a1, a2, ... 和 b1, b2, ... 不同。

\n\n

注意：

\n\n\n\n

 

\n\n

示例 1：

\n\n
\n输入：s = 'bccb'\n输出：6\n解释：6 个不同的非空回文子字符序列分别为：'b', 'c', 'bb', 'cc', 'bcb', 'bccb'。\n注意：'bcb' 虽然出现两次但仅计数一次。\n
\n\n

示例 2：

\n\n
\n输入：s = 'abcdabcdabcdabcdabcdabcdabcdabcddcbadcbadcbadcbadcbadcbadcbadcba'\n输出：104860361\n解释：共有 3104860382 个不同的非空回文子序列，104860361 对 109 + 7 取模后的值。\n
\n\n

 

\n\n

提示：

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