Manacher's Algorithm - Linear Time Longest Palindromic Substring - Part 3

// A C# program to implement Manacher’s Algorithm
using System;
using System.Diagnostics;
public class GFG
{

public static void findLongestPalindromicString(
                   String text)
{
    int N = text.Length;
    if(N == 0)
        return;
    
    // Position count
    N = 2 * N + 1; 
    
    // LPS Length Array
    int []L = new int [N]; 
    L[0] = 0;
    L[1] = 1;
    
    // centerPosition 
    int C = 1;
    
    // centerRightPosition
    int R = 2; 
    
    // currentRightPosition
    int i = 0; 
    
    // currentLeftPosition
    int iMirror; 
    int expand = -1;
    int diff = -1;
    int maxLPSLength = 0;
    int maxLPSCenterPosition = 0;
    int start = -1;
    int end = -1;
    
    // Uncomment it to print LPS Length array
    // printf("%d %d ", L[0], L[1]);
    for (i = 2; i < N; i++) 
    {
        // Get currentLeftPosition iMirror
        // for currentRightPosition i
        iMirror = 2 * C - i;
        
        // Reset expand - means no 
        // expansion required
        expand = 0;
        diff = R - i;
        
        // If currentRightPosition i is 
        // within centerRightPosition R
        if(diff >= 0) 
        {
            
            // Case 1
            if(L[iMirror] < diff) 
                L[i] = L[iMirror];
                
            // Case 2
            else if(L[iMirror] == diff && R == N - 1) 
                L[i] = L[iMirror];
            
            // Case 3
            else if(L[iMirror] == diff && R < N - 1) 
            {
                    L[i] = L[iMirror];
                    
                     // Expansion required
                    expand = 1;
            }
            
            // Case 4
            else if(L[iMirror] > diff) 
            {
                L[i] = diff;
                
                // Expansion required
                expand = 1; 
            }
        }
        else
        {
            L[i] = 0;
            
            // Expansion required
            expand = 1; 
        }
        
        if (expand == 1)
        {
            
            // Attempt to expand palindrome centered
            // at currentRightPosition i. Here for odd
            // positions, we compare characters and 
            // if match then increment LPS Length by ONE
            // If even position, we just increment LPS
            // by ONE without any character comparison
            try
            {
                while (((i + L[i]) < N && 
                        (i - L[i]) > 0) &&
                      (((i + L[i] + 1) % 2 == 0) ||
                       (text[(i + L[i] + 1) / 2] == 
                        text[(i - L[i] - 1) / 2])))
                {
                    L[i]++;
                }
            }
            catch (Exception)
            {
                Debug.Assert(true);
            }
        }
        
        // Track maxLPSLength
        if(L[i] > maxLPSLength) 
        {
            maxLPSLength = L[i];
            maxLPSCenterPosition = i;
        }

        // If palindrome centered at 
        // currentRightPosition i expand
        // beyond centerRightPosition R,
        // adjust centerPosition C based
        // on expanded palindrome.
        if (i + L[i] > R) 
        {
            C = i;
            R = i + L[i];
        }
        
        //Uncomment it to print LPS Length array
        //System.out.print("%d ", L[i]);
    }
    
    start = (maxLPSCenterPosition - 
             maxLPSLength) / 2;
    end = start + maxLPSLength - 1;
    
    //System.out.print("start: %d end: %d\n", 
    //                  start, end);
    Console.Write("LPS of string is " + 
                      text + " : ");
    
    for(i = start; i <= end; i++)
        Console.Write(text[i]);
    Console.WriteLine();
}

// Driver code
static public void Main ()
{

    String text1 = "babcbabcbaccba";
    findLongestPalindromicString(text1);

    String text2 = "abaaba";
    findLongestPalindromicString(text2);

    String text3 = "abababa";
    findLongestPalindromicString(text3);

    String text4 = "abcbabcbabcba";
    findLongestPalindromicString(text4);

    String text5 = "forgeeksskeegfor";
    findLongestPalindromicString(text5);

    String text6 = "caba";
    findLongestPalindromicString(text6);

    String text7 = "abacdfgdcaba";
    findLongestPalindromicString(text7);

    String text8 = "abacdfgdcabba";
    findLongestPalindromicString(text8);

    String text9 = "abacdedcaba";
    findLongestPalindromicString(text9);
}
}

// This code is contributed by Dharanendra L V.