Medium
Given a string s, return the number of palindromic substrings in it.
A string is a palindrome when it reads the same backward as forward.
A substring is a contiguous sequence of characters within the string.
Example 1:
Input: s = "abc"
Output: 3
Explanation: Three palindromic strings: "a", "b", "c".
Example 2:
Input: s = "aaa"
Output: 6
Explanation: Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa".
Constraints:
1 <= s.length <= 1000sconsists of lowercase English letters.
(define (check-polin vec left right)
(cond [(< left 0) 0]
[(>= right (vector-length vec)) 0]
[(not (eq? (vector-ref vec left) (vector-ref vec right))) 0]
[else (add1 (check-polin vec (sub1 left) (add1 right)))]))
(define (count-substrings s)
(let ([vec (list->vector (string->list s))]
[len (string-length s)])
(apply + (for/list ([i (range len)])
(+ 1
(check-polin vec (sub1 i) (add1 i))
(check-polin vec (sub1 i) i))))))