longestPalindromicSubstring.js

function expandAroundCenter (test, left, right) {
	/*
	:param test: Input string whose substrings are to be checked
	:param left: Left end of result palindromic substring
	:param right: Right end of result palindromic substring
	:return: Length of palindromic substring
	*/
	let n = test.length;
	while (left >= 0 && right < n && test.charAt(left) === test.charAt(right)) {
		left--;
		right++;
	}
	return right - left - 1;
}

function longestPalindromicSubstring (test) {
	/*
	Function to find longest substring which is a palindrome

	:param test: Input string whose substrings are to be checked
	:return: Longest substring of input string which is a palindrome

	Time complexity: O(n^2)
	Space complexity: O(1)
	*/
	let start = 0;
	let end = 0;

	for (let i = 0; i < test.length; i++) {
		let length = Math.max(expandAroundCenter(test, i, i),
													expandAroundCenter(test, i, i + 1));
		if (length > end - start) {
			start = i - Math.floor((length - 1) / 2);
			end = i + Math.floor(length / 2);
		}
	}
	return test.slice(start, end + 1);
}

function main () {
	let test = 'geeksforgeeks';
	console.log('Longest Palindromic Substring of',
							test, 'is', longestPalindromicSubstring(test));
}

main();