Palindromic Substrings

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 <= 1000
  • s consists of lowercase English letters.
SOLUTION:
class Solution:
    def countSubstrings(self, s: str, i = 0) -> int:
        n = len(s)
        if i == n - 1:
            return 1
        k = self.countSubstrings(s, i + 1)
        ctr = 0
        for j in range(i + 1, n + 1):
            if s[i:j] == s[i:j][::-1]:
                ctr += 1
        return ctr + k

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