longest palindromic substring

def (item):
        return item[1] - item[0]
class Solution:
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        if len(s) is 0: return ''
        if len(s) is 1: return s
        p = {}
        for i in range(len(s)):
            if s[i] in p.keys():
                p[s[i]].append(i)
            else:
                p[s[i]] = [i]
        pairs = []
        for i in p.values():
            if len(i) > 1:
                for j in range(len(i)-1):
                    for k in range(1,len(i)):
                        pairs.append([i[j],i[k]])
        if pairs == []: return s[0]
        
        #     for i in range(20):
        #         large = max(pairs,key=minus)
        #         subs = s[large[0]:large[1]+1]
        #         if subs == subs[::-1]:
        #             return subs
        #         pairs.remove(large)
        pairs.sort(key=minus, reverse = True)
        for i in pairs:
            subs = s[i[0]:i[1]+1]
            if subs == subs[::-1]:
                return subs
        return s[0]

class Solution:
    def longestPalindrome(self, s):
        '''
        :type s: str
        :rtype: str
        '''
        if len(s)<=1: return s
        start,left,right=0,0,0 # init
        while (start+int((right-left)/2)<len(s)):
            i,j=0,1 # two pointer, i - repeat chars, j - palindrome step
            while (start+i<len(s)-1): # find the repeat chars amount start+i+1<=len(s)-1
                if s[start]==s[start+i+1]: i+=1
                else: break  
            while(start-j>=0 and start+i+j<len(s)): # find the number of palindrome step
                if s[start-j]==s[start+i+j]:j+=1
                else: break
            if (right-left<i-1+2*j):left,right=start-j+1,start+j+i
            start=start+i+1
        return s[left:right]