730. Count Different Palindromic Subsequences
Hard47.5% acceptance44,757 / 94,191 submissions
Asked by 5 companies
Topics
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.
Example 1:
Input: s = "bccb" Output: 6 Explanation: The 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.
Example 2:
Input: s = "abcdabcdabcdabcdabcdabcdabcdabcddcbadcbadcbadcbadcbadcbadcbadcba" Output: 104860361 Explanation: There 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'.
Hints
Hint 1
Let dp(i, j) be the answer for the string T = S[i:j+1] including the empty sequence. The answer is the number of unique characters in T, plus palindromes of the form "a_a", "b_b", "c_c", and "d_d", where "_" represents zero or more characters.