LeetCode in Elixir

5. Longest Palindromic Substring

Medium

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

Example 1:

Input: s = “babad”

Output: “bab” Note: “aba” is also a valid answer.

Example 2:

Input: s = “cbbd”

Output: “bb”

Example 3:

Input: s = “a”

Output: “a”

Example 4:

Input: s = “ac”

Output: “a”

Constraints:

• 1 <= s.length <= 1000
• s consist of only digits and English letters.

Solution

defmodule Solution do
  @spec longest_palindrome(s :: String.t()) :: String.t()
  def longest_palindrome(<<>>), do: ""
  def longest_palindrome(<<f::binary-size(1)>>), do: f
  def longest_palindrome(s) do
    {ri,rj} =String.to_charlist(s) |> List.to_tuple |>  do_longest_palindrome(0, {0,0})
    String.slice(s,ri..rj)
  end

  defp do_longest_palindrome(s_t,i,{ri,rj}) when i == tuple_size(s_t), do: {ri,rj}
  defp do_longest_palindrome(s_t,i,{ri,rj}) do
    {ri,rj} = do_longest_palindrome(s_t,i,i,{ri,rj})
    {ri,rj} = do_longest_palindrome(s_t,i,i+1,{ri,rj})
    do_longest_palindrome(s_t, i+1, {ri, rj})
  end

  defp do_longest_palindrome(s_t,i,j,{ri,rj}) when (i<0 or j==tuple_size(s_t)) or (elem(s_t,i) != elem(s_t,j)) do
    if ((j-1)-(i+1)) > rj-ri, do: {i+1,j-1}, else: {ri,rj}
  end
  defp do_longest_palindrome(s_t,i,j,{ri,rj}), do: do_longest_palindrome(s_t, i-1, j+1, {ri,rj})
end

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