Program to Find the Longest Repeating Sequence in a String

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Q. Program to find the longest repeating sequence in a string.

Explanation

In this program, we need to find the substring which has been repeated in the original string more than once.

In the above string, the substring bdf is the longest sequence which has been repeated twice.

Algorithm

Define a string and calculate its length.
Define a function for the longest common prefix that is, it takes two strings as arguments and determines the longest group of characters common in between them.
Using the same function, we will compare original string will all its substrings. Then, first for loop compare all the substrings with all the other substrings till we find the longest repeating sequence.
Storing the longest string in the variable lrs if the length of x is greater than lrs.

Solution

Python

#Checks for the largest common prefix
def lcp(s, t):
  n = min(len(s),len(t));
  for i in range(0,n):
    if(s[i] != t[i]):
      return s[0:i];
  else:
    return s[0:n];
 
str = "acbdfghybdf";
lrs="";
n = len(str);
for i in range(0,n):
  for j in range(i+1,n):
    #Checks for the largest common factors in every substring
    x = lcp(str[i:n],str[j:n]);
        #If the current prefix is greater than previous one 
        #then it takes the current one as longest repeating sequence
    if(len(x) > len(lrs)):
      lrs=x;  
print("Longest repeating sequence: "+lrs);

Output:

Longest repeating sequence: bdf

C

#include <stdio.h>
#include <string.h>
#define MIN(x, y) (((x) < (y)) ? (x) : (y))
//User-defined substring function that will take string(str), position(p) and no of character(len) as input
//Produces result c as output
void substring(char s[], char sub[], int p, int len){
 int c = 0;
 while (c < len) {
 sub[c] = s[p+c];
  c++;
}
 sub[c] = '\0';
}

//Function thats gives the longest common prefix among two strings.
int lcp(char s[], char t[],char a[]){
 int n = MIN(strlen(s),strlen(t));
for(int i = 0; i < n; i++){
  if(s[i] != t[i]){
   substring(s,a,0,i);
    return 0;
 }
}
substring(s,a,0,n);
return 0;
}

int main()
{
  char str[] = "acbdfghybdf";
char lrs[30], x[30], res[30], sub[30],sub1[30];
int i,j,n = strlen(str);
for(i = 0; i < n; i++){
 for(j = i+1; j < n; j++){
    //For comparing each substring with every other substring
   substring(str,sub,i,n);
        substring(str,sub1,j,n);
   cp(sub,sub1,x);
    //lrs keeps track of the longest repeating sequence
    if(strlen(x) > strlen(lrs)) strncpy(lrs, x, strlen(x));
 }
}
printf("Longest repeating sequence: %s",lrs);
  return 0;
}

Output:

Longest repeating sequence: bdf

JAVA

public class LongestRepeatingSequence {
    //Checks for the largest common prefix
    public static String lcp(String s, String t){
        int n = Math.min(s.length(),t.length());
        for(int i = 0; i < n; i++){
            if(s.charAt(i) != t.charAt(i)){
                return s.substring(0,i);
            }
        }
        return s.substring(0,n);
    }
    
    public static void main(String[] args) {
        String str = "acbdfghybdf";
        String lrs="";
        int n = str.length();
        for(int i = 0; i < n; i++){
            for(int j = i+1; j < n; j++){
                //Checks for the largest common factors in every substring
                String x = lcp(str.substring(i,n),str.substring(j,n));
                //If the current prefix is greater than previous one 
                //then it takes the current one as longest repeating sequence
                if(x.length() > lrs.length()) lrs=x;  
            }
        }
        System.out.println("Longest repeating sequence: "+lrs);
    }
}

Output:

Longest repeating sequence: bdf

C#

using System;                 
public class Program
{
    //Checks for the largest common prefix
    public static String lcp(String s, String t){
        int n = Math.Min(s.Length,t.Length);
        for(int i = 0; i < n; i++){
            if(s[i] != t[i]){
                return s.Substring(0,i);
            }
        }
        return s.Substring(0,n);
    }
    
    public static void Main()
    {
        String str = "acbdfghybdf";
        String lrs="";
        int n = str.Length;
        for(int i = 0; i < n; i++){
           for(int j = i+1; j < n-i; j++){
                //Checks for the largest common factors in every substring
                String x = lcp(str.Substring(i),str.Substring(j));
                //If the current prefix is greater than previous one 
                //then it takes the current one as longest repeating sequence
               if(x.Length > lrs.Length) lrs=x;  
            }
        }
       Console.WriteLine("Longest repeating sequence: "+lrs);
    }
}

Output:

Longest repeating sequence: bdf

PHP

<!DOCTYPE html> 
<html> 
<body> 
<?php 
//Checks for the largest common prefix
function lcp($s, $t){
    $n = min(strlen($s),strlen($t));
    for($i = 0; $i < $n; $i++){
        if($s[$i] != $t[$i]){
            return substr($s,0,$i);
        }
    }
    return substr($s,0,$n);
}
    
$str = "acbdfghybdf";
$lrs="";
$n = strlen($str);
for($i = 0; $i < $n; $i++){
    for($j = ($i+1); $j < ($n-$i); $j++){
        //Checks for the largest common factors in every substring
        $x = lcp(substr($str,$i),substr($str,$j));
        //If the current prefix is greater than previous one 
        //then it takes the current one as largest repeating sequence
        if(strlen($x) > strlen($lrs))
        { 
            $lrs=$x;
        }  
    }
}
echo "Longest repeating sequence: $lrs";
?> 
</body> 
</html>

Output: