Longest Palindromic Substring in Python

Suppose we have a string S. We have to find the longest palindromic substring in S. We are assuming that the length of the string S is 1000. So if the string is “BABAC”, then the longest palindromic substring is “BAB”.

To solve this, we will follow these steps

• Define one square matrix of order same as the length of string, and fill it with False
• Set the major diagonal elements as true, so DP[i, i] = True for all i from 0 to order – 1
• start := 0
• for l in range 2 to length of S + 1
  • for i in range 0 to length of S – l + 1
    • end := i + l
    • if l = 2, then
      • if S[i] = S[end - 1], then
        • DP[i, end - 1] = True, max_len := l, and start := i
    • otherwise
      • if S[i] = S[end - 1] and DP[i + 1, end - 2], then
        • DP[i, end - 1] = True, max_len := l, and start := i
• return a substring of from index start to start + max_len

Example(Python)

Let us see the following implementation to get better understanding −

class Solution(object):
   def longestPalindrome(self, s):
      dp = [[False for i in range(len(s))] for i in range(len(s))]
      for i in range(len(s)):
         dp[i][i] = True
      max_length = 1
      start = 0
      for l in range(2,len(s)+1):
         for i in range(len(s)-l+1):
            end = i+l
            if l==2:
               if s[i] == s[end-1]:
                  dp[i][end-1]=True
                  max_length = l
                  start = i
            else:
               if s[i] == s[end-1] and dp[i+1][end-2]:
                  dp[i][end-1]=True
                  max_length = l
                  start = i
      return s[start:start+max_length]
ob1 = Solution()
print(ob1.longestPalindrome("ABBABBC"))

Input

"ABBABBC"

Output

"BBABB"