Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

def longestPalindrome(S):
   if len(S) == 0 or len(S) < 2:
      return S
   dp = [[False for _ in range(len(S))] for _ in range(len(S))]
   res = ''
   maxlen = 0
   dp[0][0] = True  j = 0
   while j < len(S):
      dp[j][j] = True  i = 0
      while i < j:
         dp[i][j] = (j - i < 2 or dp[i + 1][j - 1]) and S[i] == S[j]
         if dp[i][j] and j-i+1 > maxlen:
            res = S[i:j+1]
            maxlen = j-i+1
         i += 1
      j += 1
   return res
print longestPalindrome("etretretdaadaad")