LC 647 - Palindromic Substrings

Question

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.

Links

Question here and solution here

Solution

concept

For the two pointer solution, it is quite similar to [[5. Longest Palindromic Substring]].

code

class Solution:
    def count_palin(self, s, l, r):
        result =  0
        while l >= 0 and r < len(s) and s[l] == s[r]:
            result += 1
            l -= 1
            r += 1
        return result

    def countSubstrings(self, s: str) -> int:
        result = 0

        for i in range(len(s)):
            l, r = i, i
            result += self.count_palin(s, l, r)

            l, r = i, i + 1
            result += self.count_palin(s,l, r)

        return result

Complexity

time: $O(n^2)$
space: $O(1)$