#2484
Count Palindromic Subsequences
hard · verified · 41% accepted · 609 likes · top 21%
Description
Given a string of digits s, return the number of palindromic subsequences of s having length 5. Since the answer may be very large, return it modulo 109 + 7.
Note:
- A string is palindromic if it reads the same forward and backward.
- A subsequence is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.
Example 1:
Input: s = "103301"
Output: 2
Explanation:
There are 6 possible subsequences of length 5: "10330","10331","10301","10301","13301","03301".
Two of them (both equal to "10301") are palindromic.
Example 2:
Input: s = "0000000"
Output: 21
Explanation: All 21 subsequences are "00000", which is palindromic.
Example 3:
Input: s = "9999900000"
Output: 2
Explanation: The only two palindromic subsequences are "99999" and "00000".
Solution