Version: v1.6.5 Latest Latest Go to latest
Published: Mar 23, 2021 License: MIT

## README ¶

### 730. Count Different Palindromic Subsequences (Hard)

Given a string S, find the number of different non-empty palindromic subsequences in S, and return that number modulo `10^9 + 7`.

A subsequence of a string S is obtained by deleting 0 or more characters from S.

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

Two sequences `A_1, A_2, ...` and `B_1, B_2, ...` are different if there is some `i` for which `A_i != B_i`.

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:
Output: 104860361
Explanation:
There are 3104860382 different non-empty palindromic subsequences, which is 104860361 modulo 10^9 + 7.
```

Note:

• The length of `S` will be in the range `[1, 1000]`.
• Each character `S[i]` will be in the set `{'a', 'b', 'c', 'd'}`.
• #### Similar Questions

1. Longest Palindromic Subsequence (Medium)

#### 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.

## Documentation ¶ There is no documentation for this package.