Add Longest Palindromic Substring in Ruby

archive/r/ruby/longest-palindromic-substring.rb

@@ -0,0 +1,28 @@
+def expand_from_center(s, left, right)
+  while left >= 0 && right < s.length && s[left] == s[right]
+    left -= 1
+    right += 1
+  end
+  s[(left + 1)...right]
+end
+
+def longest_palindromic_substring(s)
+  abort "Usage: please provide a string that contains at least one palindrome" if s.to_s.empty?
+
+  longest = ""
+
+  s.length.times do |i|
+    odd  = expand_from_center(s, i, i)
+    even = expand_from_center(s, i, i + 1)
+
+    longest = odd  if odd.length  > longest.length
+    longest = even if even.length > longest.length
+  end
+
+  abort "Usage: please provide a string that contains at least one palindrome" if longest.length <= 1
+
+  puts longest
+end
+
+input = ARGV.first
+longest_palindromic_substring(input)