KMP Agorithm (Part 1 : Constructing LPS Array)

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Tue Feb 08 2022 12:02:37 GMT+0000 (UTC)

Saved by @Uttam #java #gfg #geeksforgeeks #lecture #string #patternsearching #kmp #lps

// Efficient Code O(n)

import java.util.*;
import java.io.*;
  
class GFG { 
  
    static void fillLPS(String str, int lps[])
    {
        int n=str.length(),len=0;
        lps[0]=0;
        int i=1;
        while(i<n){
            if(str.charAt(i)==str.charAt(len))
            {len++;lps[i]=len;i++;}
            else
            {if(len==0){lps[i]=0;i++;}
                else{len=lps[len-1];}
            }
        }
    }
  
    public static void main(String args[]) 
    {   String txt = "abacabad";int[] lps=new int[txt.length()];
        fillLPS(txt,lps);
        for(int i=0;i<txt.length();i++){
            System.out.print(lps[i]+" ");
        } 
    } 
} 




// Naive Code O(n^3)

import java.util.*;
import java.io.*;
  
class GFG { 

    static int longPropPreSuff(String str, int n)
    {
        for(int len=n-1;len>0;len--){
            boolean flag=true;
            for(int i=0;i<len;i++)
                if(str.charAt(i)!=str.charAt(n-len+i))
                    flag=false;
                    
            if(flag==true)
                return len;
        }
        return 0;
    }

    static void fillLPS(String str, int lps[]){
        for(int i=0;i<str.length();i++){
        lps[i]=longPropPreSuff(str,i+1);
        }
    }
  
    public static void main(String args[]) 
    {   String txt = "abacabad";int[] lps=new int[txt.length()];
        fillLPS(txt,lps);
        for(int i=0;i<txt.length();i++){
            System.out.print(lps[i]+" ");
    }  
    } 
} 
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Two methods of LPS (Longest Proper Prefx Suffix) Array are discussed. One method has time complexity O(n^3) and other method is O(n).