# LeetCode 5. Longest Palindromic Substring

## Description

https://leetcode.com/problems/longest-palindromic-substring/description/

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

```Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.
```

Example 2:

```Input: "cbbd"
Output: "bb"```

## Explanation

A palindromic string meets two criteria:

1. first and the last character are the same
2. the inner word between the first and the last character is a palindromic string

The basic idea is to use dynamic programming to determine whether a substring is palindromic string base on above criteria.

Need to be careful that when returning the result, the end index need to plus 1, because java substring method doesn’t include the ending index.

## Java Solution

``````class Solution {
public String longestPalindrome(String s) {
if (s == null || s.length() < 2) {
return s;
}

int length = s.length();

boolean[][] isPalindrome = new boolean[length][length];

int left = 0;
int right = 0;

for (int j = 1; j < length; j++) {
for (int i = 0; i < j; i++) {
boolean isInnerWordPalindrome = isPalindrome[i + 1][j - 1] || j - i <= 2;

if (s.charAt(i) == s.charAt(j) && isInnerWordPalindrome) {
isPalindrome[i][j] = true;

if (j - i > right - left) {
left = i;
right = j;
}
}
}
}

return s.substring(left, right + 1);

// Time Complexity: O(n ^ 2)
// Space Complexity: O(n ^ 2)
}
}``````

## Python Solution

``````class Solution:
def longestPalindrome(self, s: str) -> str:
if not s:
return ""

longest = ""
for middle in range(len(s)):
sub = self.find_palindrome_from(s, middle, middle)
if len(sub) > len(longest):
longest = sub

sub = self.find_palindrome_from(s, middle, middle + 1)
if len(sub) > len(longest):
longest = sub

return longest

def find_palindrome_from(self, string, left, right):
while left >= 0 and right < len(string):
if string[left] != string[right]:
break
left -= 1
right += 1

return string[left + 1:right]        ``````
• Time complexity: O(N^2).
• Space complexity: O(S).

## 5 Thoughts to “LeetCode 5. Longest Palindromic Substring”

1. Jason says:

It’s interesting how you do the double for loop
The way how you set up right now is “i” will never exceed “j” and “i” is the inner loop

let me rename “i” and “j” for clarity sake i = left and j = right
the way how you set up after renaming looks like this

for (right = 1; right < length; right ++) {
for (left = 0; left < right; left ++) {
* …. *
}
}

just curious why?
vs something like this

for (left = 0; left < length – 1; left++ ) {
for (right = (left + 1); right < length; right ++ ) {
* …. *
}
}

Correct me if I am wrong, I print out substring with i, j index and I think both of them are the same.
In my opinion the second for-loop feels more intuitive unless this will cause an issue building out the "isInnerWordPalindrome" matrix

thoughts?

2. Divya Singh says:

I have the same question in python but the platform accepts input only in this format. The problem is Test Case with 12 as length and abcbcabbacba as string is resulting in 12 and same string as output , it should be 8, bcabbacb. Can you tell where I misunderstood the logic and syntax. My code is as follows:

#input length, string
N=int(input())
w=input()

if w == ” or len(w)<=2:
print(len(w))
print(w)

if N<=5000:
isPallindrome = [*N]*N
left=0
right=0

for j in range(1,N):
for i in range(0,j):
isInnerWordPallindrome = isPallindrome[i+1][j-1] or j-i right-left:
left = i
right = j

var1 = len(w[left:right+1])
var2 = w[left:right+1]
print(var1)
print(var2)

3. Mohd Shahbaz Alam says:

Can you please explain why are you taking j from 1 to length and not from 0 to length inside the first loop?
Please elaborate both the ranges for i and j.

1. GoodTecher says: