LeetCode-in-All

647. Palindromic Substrings

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 <= 1000
• s consists of lowercase English letters.

Solution

class Solution:
    def expand(self, a: List[str], l: int, r: int, res: List[int]) -> None:
        while l >= 0 and r < len(a):
            if a[l] != a[r]:
                return
            else:
                res[0] += 1
                l -= 1
                r += 1

    def countSubstrings(self, s: str) -> int:
        a = list(s)
        res = [0]
        for i in range(len(a)):
            self.expand(a, i, i, res)
            self.expand(a, i, i + 1, res)
        return res[0]

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