longest_palindrome.rb

require 'pry'
require 'pp'

def get_panlindrome(string, start, tail)
  while string[start] == string[tail] && start >= 0 && tail < string.length
    start -= 1
    tail += 1
  end
  string[start + 1..tail - 1]
end

# Running time is O(n^2)

# sudo code:

# 1. return string if the length is one.
# 2. initialize the longest palindrome as the first letter.
# 3. iterate each letter from the beginning. 
#    case 1) treat each letter as the pivot, we want to compare letter both from left and right.
#    case 2) treat each letter and the following as the povit, we want to compare letter both from left and right.
#    keep moving the pointers until value of letter does not match
# 4. replace the palindrome with the longest returning value.

def longest_palindrome(string)
  length = string.length
  return string if length == 1

  longest = string[0]
  for i in 0..length - 1

    palindrome = get_panlindrome(string, i, i)

    if palindrome.length > longest.length
      longest = palindrome
    end

    palindrome = get_panlindrome(string, i, i+1)

    if palindrome.length > longest.length
      longest = palindrome
    end

  end
  longest
end

pp longest_palindrome('aba')
pp longest_palindrome('abadd')
pp longest_palindrome('abba')
pp longest_palindrome('sadfjhdsakjfabba')
pp longest_palindrome('ddddffddddabba')