Manachers_Algorithm.java

import java.util.Scanner;

public Manachers_Algorithm{
 private static final int SIZE = 100000 + 1;
 private static int[] P = new int[SIZE * 2];

 // Transform S into new string Q with special characters inserted to avoid bound checking.
 private static String convertToNewString(String s) {
 StringBuilder newString = new StringBuilder("@");

 for (int i = 0; i < s.length(); i++) {
 newString.append("#").append(s.charAt(i));
 }

 newString.append("#$");
 return newString.toString();
 }

 private static String longestPalindromeSubstring(String s) {
 String Q = convertToNewString(s);
 int c = 0, r = 0; // current center, right limit

 for (int i = 1; i < Q.length() - 1; i++) {
 // find the corresponding letter in the palindrome substring
 int iMirror = c - (i - c);

 if (r > i) {
 P[i] = Math.min(r - i, P[iMirror]);
 }

 // expanding around center i
 while (Q.charAt(i + 1 + P[i]) == Q.charAt(i - 1 - P[i])) {
 P[i]++;
 }

 // Update c,r in case if the palindrome centered at i expands past r
 if (i + P[i] > r) {
 c = i; // next center = i
 r = i + P[i];
 }
 }

 // Find the longest palindrome length in p
 int maxPalindrome = 0;
 int centerIndex = 0;

 for (int i = 1; i < Q.length() - 1; i++) {
 if (P[i] > maxPalindrome) {
 maxPalindrome = P[i];
 centerIndex = i;
 }
 }

 return s.substring((centerIndex - 1 - maxPalindrome) / 2, (centerIndex - 1 - maxPalindrome) / 2 + maxPalindrome);
 }

 public static void main(String[] args) {
 Scanner scanner = new Scanner(System.in);
 String s = scanner.next();
 System.out.println(longestPalindromeSubstring(s));
 }
}