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 countSubstrings(self, s: str) -> int:
        result = 0
        i = 0
        while i < len(s):
            l = i
            while i < len(s) and s[i] == s[l]:
                i+=1
            r = i - 1
            for j in range(r-l+1):
                result += j+1
            while l-1>=0 and r+1 < len(s) and s[l-1] == s[r+1]:
                result+=1

                l-=1
                r+=1
        return result

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