5. Longest Palindromic Substring

Given a string s, return the longest palindromic substring in s.

Input: s = "babad"
Output: "bab"
Explanation: "aba" is also a valid answer.

Input: s = "cbbd"
Output: "bb"

Solution

1. Loop through each character of s
2. Check if s[i] or s[i] + s[i + 1] is center of a palindromic

function longestPalindrome(s: string): string {
    if (!s || s.length <= 1) { return s }
    let longestPalindrome = s.substring(0, 1)

    for (let i = 0; i < s.length; i++) {
        [expand(s, i, i), expand(s, i, i + 1)].forEach((maybeLongest) => {
            if (maybeLongest.length > longestPalindrome.length) {
                longestPalindrome = maybeLongest
            }
        })
    }

    return longestPalindrome
}

function expand(s: string, begin: number, end: number): string {
    while (begin >= 0 && end <= s.length - 1 && s[begin] === s[end]) {
        begin--
        end++
    }

    return s.substring(begin + 1, end)
}