Count Palindromic Subsequences - Video Solutions

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".

 

Constraints:

• 1 <= s.length <= 104
• s consists of digits.