Python 3.8 (Coding): Longest Palindromic Substring Test

The longestPalindrome solution returns the longest palindromic substring found in s. Initializes variables n, start, and max_len. n represents the length of the input string s. start will store the starting index of the longest palindromic substring found so far. max_len will store the length of the longest palindromic substring found so far. It is initially set to 1 if n is greater than 0, representing the case when s has at least one character. If n is 0, max_len remains 0. The solution uses a nested loop to iterate through all possible positions i in the string s. The outer loop iterates from left to right, considering each character in s.

Inside the outer loop, the first inner loop checks for palindromic substrings with an odd length centered at position i. It uses two pointers, l and r, initially set to i-1 and i, respectively. The loop continues as long as l is greater than or equal to 0 and r is less than n, and the characters at positions l and r are equal. This loop expands the palindromic substring outward from position i.

If the length of the palindromic substring found (r - l + 1) is greater than max_len, it means we have found a longer palindromic substring. In this case, we update max_len to the new length and store the starting index of the substring in start.

The second inner loop checks for palindromic substrings with an even length centered at position i. It uses two pointers, l and r, initially set to i-1 and i+1, respectively. The loop continues as long as l is greater than or equal to 0 and r is less than n, and the characters at positions l and r are equal. This loop also expands the palindromic substring outward from position i.

Again, if the length of the palindromic substring found (r - l + 1) is greater than max_len, we update max_len and store the starting index of the substring in start.

After the nested loops finish, we check if max_len is still 0. If it is, it means no palindromic substring was found in s, so we return an empty string.

Otherwise, we use the substr function to extract the longest palindromic substring from s, starting from index start and having a length of max_len. We then return this substring as the output of the function.