How to Solve Count Different Palindromic Subsequences Problem
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Count Different Palindromic Subsequences
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. A subsequence of a string is obtained by deletin...
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Understanding the Count Different Palindromic Subsequences Problem
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Problem Statement
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. A subsequence of a string is obtained by deleting zero or more characters from the string. A sequence is palindromic if it is equal to the sequence reversed. Two sequences a1, a2, ... and b1, b2, ... are different if there is some i for which ai != bi.
Count Different Palindromic Subsequences
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Examples
s = "bccb"6The 6 different non-empty palindromic subsequences are 'b', 'c', 'bb', 'cc', 'bcb', 'bccb'. Note that 'bcb' is counted only once, even though it occurs twice.
s = "abcdabcdabcdabcdabcdabcdabcdabcddcbadcbadcbadcbadcbadcbadcbadcba"104860361There are 3104860382 different non-empty palindromic subsequences, which is 104860361 modulo 109 + 7.
Constraints
1 <= s.length <= 1000s[i] is either 'a', 'b', 'c', or 'd'.How Interview Coder Helps with Leetcode Problems
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