Leetcode - Longest Palindromic Substring Solution
1 min read

Leetcode - Longest Palindromic Substring Solution

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

Example 1:

Input: s = "babad"
Output: "bab"
Explanation: "aba" is also a valid answer.

Example 2:

Input: s = "cbbd"
Output: "bb"

Constraints:

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

Solution in Python3

class Solution:
    def longestPalindrome(self, s: str) -> str:
        if len(s)==1:
            return s
        def createCenter(s,mid):
            yield mid
            for i in range(1,mid+1):
                yield mid-i
                if mid+i == len(s):
                    continue
                yield mid+i
        def longestPalin(s,index):
            i = index-1
            j = index+1
            while i>-1 and j<len(s) and s[i]==s[j]:
                i-=1
                j+=1
            return s[i+1:j]

        def longestPalinDual(s,i,j):
            while i>-1 and j<len(s) and s[i]==s[j]:
                i-=1
                j+=1
            return s[i+1:j]

        mid = len(s)//2
        longest = ""
        for center in createCenter(s,mid):
            l = center
            r = len(s) - center - 1
            lp = min(l,r)*2+1
            pal = longestPalin(s,center)
            if len(pal)>len(longest):
                longest = pal
            if lp<=len(longest):
                #print(longest)
                break
            #print(center,len(longestPalin(s,center)),lp,longest)
        # DUAL
        centers = []
        for i in range(1,mid+1):
            centers.extend([(mid-i,mid-i+1),(mid+i-1,mid+i)])
        # DUAL
        if centers[-1][-1]==len(s):
            centers.pop()
        # DUAL
        longest_dual = ""
        for center in centers:
            x,y = center
            l = x
            r = len(s) - y - 1
            lp = min(l,r)*2+2
            if s[x] == s[y]:
                pal = longestPalinDual(s,x,y)
                if len(pal)>len(longest_dual):
                    longest_dual = pal
                if lp<=len(longest_dual):
                    #print(longest_dual)
                    break
        if len(longest)>len(longest_dual):
            return longest
        else:
            return longest_dual

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