Ruby最长回文

def longest_palindrome(s)
  return s if s.length < 2

  start = 0
  max_len = 0

  (0...s.length).each do |i|
    expand(s, i, i, start, max_len) # 奇数长度的回文串
    expand(s, i, i + 1, start, max_len) # 偶数长度的回文串
  end

  s[start...(start + max_len)]
end

def expand(s, left, right, start, max_len)
  while left >= 0 && right < s.length && s[left] == s[right]
    left -= 1
    right += 1
  end

  len = right - left - 1
  if len > max_len
    start = left + 1
    max_len = len
  end
end

# 示例用法
puts longest_palindrome("babad") # 输出 "bab"
puts longest_palindrome("cbbd") # 输出 "bb"