Palindrome DP

function longestPalindrome(s: string): string {
  let start = 0, maxLen = 0;
  const expand = (l: number, r: number): void => {
    while (l >= 0 && r < s.length && s[l] === s[r]) { l--; r++; }
    const len = r - l - 1;
    if (len > maxLen) { maxLen = len; start = l + 1; }
  };
  for (let i = 0; i < s.length; i++) {
    expand(i, i);      // odd-length center
    expand(i, i + 1);  // even-length center
  }
  return s.substring(start, start + maxLen);
}

def longest_palindrome(s)
  start = 0
  max_len = 0
  expand = lambda do |l, r|
    while l >= 0 && r < s.length && s[l] == s[r]
      l -= 1
      r += 1
    end
    len = r - l - 1
    if len > max_len
      max_len = len
      start = l + 1
    end
  end
  s.length.times do |i|
    expand.call(i, i)     # odd-length center
    expand.call(i, i + 1) # even-length center
  end
  s[start, max_len]
end