The best approach is that from the mid of any palindrome, if we go to right and left by 1 place, it’s always same character. For example 12321, here mid is 3 and if we keep moving one position in both sides, we get 2 and then 1.

We will use the same logic in our java program to find out the longest palindrome.

However if the palindrome length is even, the mid size is also even, so we need to make sure in our program that this is also checked, for example 12333321, here mid is 33 and if we keep moving one position in both sides, we get 3, 2 and 1.

public class LongestPalindromeFinder {

public static void main(String[] args) {

System.out.println(longestPalindromeString("1234"));

System.out.println(longestPalindromeString("12321"));

System.out.println(longestPalindromeString("9912321456"));

System.out.println(longestPalindromeString("9912333321456"));

System.out.println(longestPalindromeString("12145445499"));

}

/**

* This method returns the longest palindrome in the input String

*

* @param in

* @return

*/

public static String longestPalindromeString(String in) {

char[] input = in.toCharArray();

int longestPalindromeStart = 0;

int longestPalindromeEnd = 0;

for (int mid = 0; mid < input.length; mid++) {

// for odd palindrom case like 12321, 3 will be the mid

int left = mid-1;

int right = mid+1;

// we need to move in the left and right side by 1 place till they reach the end

while (left >= 0 && right < input.length) {

// below check to find out if its a palindrome

if (input[left] == input[right]) {

// update global indexes only if this is the longest one till now

if (right - left > longestPalindromeEnd

- longestPalindromeStart) {

longestPalindromeStart = left;

longestPalindromeEnd = right;

}

}

left--;

right++;

}

// for even palindrome, we need to have similar logic with mid size 2

// for that we will start right from one extra place

left = mid-1;

right = mid + 2;// for example 12333321 when we choose 33 as mid

while (left >= 0 && right < input.length)

{

if (input[left] == input[right]) {

if (right - left > longestPalindromeEnd

- longestPalindromeStart) {

longestPalindromeStart = left;

longestPalindromeEnd = right;

}

}

left--;

right++;

}

}

// we have the start and end indexes for longest palindrome now

return in.substring(longestPalindromeStart, longestPalindromeEnd + 1);

}

}

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