Palindromic Substrings — LeetCode 647 Python Solution

The problem

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.

Example

Input

s = "abc"

Output

3

Explanation

Three palindromic strings: "a", "b", "c".

Python solution

class Solution:
    def countSubstrings(self, s: str) -> int:
        ans, n = 0, len(s)
        for k in range(n * 2 - 1):
            i, j = k // 2, (k + 1) // 2
            while ~i and j < n and s[i] == s[j]:
                ans += 1
                i, j = i - 1, j + 1
        return ans

Complexity

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 647. Palindromic Substrings is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Two Pointers.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

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