LeetCode 647: Palindromic Substrings — Step-by-Step Visual Trace

Medium — String | Two Pointers | Expand Around Centers | Palindrome The Problem Count the total number of palindromic substrings in a given string, where a palindromic substring reads the same forwards and backwards. Approach Use the expand around centers technique by checking every possible center position in the string. For each position, expand outward while characters match to count palindromes of both odd and even lengths. Time: O(n²) · Space: O(1) Code class Solution : def countSubstrings ( self , s : str ) -> int : def expandAroundCenter ( left : int , right : int ) -> int : count = 0 # Expand around the center while the characters at both ends are equal. while left >= 0 and right < len ( s ) and s [ left ] == s [ right ]: left -= 1 right += 1 count += 1 return count if not s : return 0 total_palindromes = 0 for i in range ( len ( s )): # Check for odd-length palindromes. total_palindromes += expandAroundCenter ( i , i ) # Check for even-length palindromes. total_palindromes +=