How to find the longest palindromic substring in java. To solve this problem, we will use 3 different ways let’s discuss them one by one

Longest Palindromic Substring in Java

we can simply examine every substring and check if it is palindromic. The time complexity is O(n3ˆ).  Let’s be the input string, I and J are two indices of the string. Define a 2-dimension array “table” and let table[i][j] denote whether substring from i to j is a palindrome.

Start condition:

table[i][i] == 1;
table[i][i+1] == 1 => s.charAt(i) == s.charAt(i+1)

Changing condition:

table[i+1][j-1] == 1 && s.charAt(i) == s.charAt(j)
table[i][j] == 1

Time O(n2ˆ) Space O(n2ˆ)

public static String longestPalindrome2(String s) {
if (s == null)
return null;
if(s.length() <=1)
return s;
int maxLen = 0;
String longestStr = null;
int length = s.length();
int[][] table = new int[length][length];
//every single letter is palindrome
for (int i = 0; i < length; i++) {
table[i][i] = 1;
}
printTable(table);
//e.g. bcba
//two consecutive same letters are palindrome
for (int i = 0; i <= length - 2; i++) {
if (s.charAt(i) == s.charAt(i + 1)){
table[i][i + 1] = 1;
longestStr = s.substring(i, i + 2);
}
}
printTable(table);
//condition for calculate whole table
for (int l = 3; l <= length; l++) {
for (int i = 0; i <= length-l; i++) {
int j = i + l - 1;
if (s.charAt(i) == s.charAt(j)) {
table[i][j] = table[i + 1][j - 1];
if (table[i][j] == 1 && l > maxLen)
longestStr = s.substring(i, j + 1);
} else {
table[i][j] = 0;
}
printTable(table);
}
}
return longestStr;
}
public static void printTable(int[][] x){
for(int [] y : x){
for(int z: y){
System.out.print(z + " ");
}
System.out.println();
}
System.out.println("------");
}

Given an input, we can use printTable method to examine the table after each iteration. For example, if input string is “dabcba”, the final matrix would be the following:
1 0 0 0 0 0
0 1 0 0 0 1
0 0 1 0 1 0
0 0 0 1 0 0
0 0 0 0 1 0
0 0 0 0 0 1
From the table, we can clearly see that the longest string is in the cell table[1][5].

public String longestPalindrome(String s) {
if (s.isEmpty()) {
return null;
}
if (s.length() == 1) {
return s;
}
String longest = s.substring(0, 1);
for (int i = 0; i < s.length(); i++) {
// get longest palindrome with center of i
String tmp = helper(s, i, i);
if (tmp.length() > longest.length()) {
longest = tmp;
}
// get longest palindrome with center of i, i+1
tmp = helper(s, i, i + 1);
if (tmp.length() > longest.length()) {
longest = tmp;
}
}
return longest;
}
// Given a center, either one letter or two letter,
// Find longest palindrome
public String helper(String s, int begin, int end) {
while (begin >= 0 && end <= s.length() - 1 && s.charAt(begin) ==
s.charAt(end)) {
begin--;
end++;
}
return s.substring(begin + 1, end);
}