Q:

Java Program to find the longest repeating sequence in a string

belongs to collection: Java String Programs

robaa
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2022-11-12
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In this program, we need to find the substring which has been repeated in the original string more than once.

A b D F A A b d f h

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

The algorithm of the program is given below.

Algorithm

main()

  • STEP 1: START
  • STEP 2: DEFINE string str = "acbdfghybdf"
  • STEP 3: SET String lrs = " "
  • STEP 4: CALCULATE length.
  • STEP 5: SET i =0. REPEAT STEP 6 to STEP 10 UNTIL i<n.
  • STEP 6: SET j =i+1. REPEAT STEP 7 to STEP 9 UNTIL j<n.
  • STEP 7: CALL lcp() in String x.
  • STEP 8: if(x.length()>lrs.length()) then lrs =x
  • STEP 9: j =j + 1
  • STEP 10: i =i +1
  • STEP 11: PRINT lrs.
  • STEP 12: END

lcp(String s, String t)

  • STEP 1: START
  • STEP 2: SET n = Math.min(s.length(), t.length())
  • STEP 3: SET i =0. REPEAT STEP 4 to STEP 5 UNTIL i<n
  • STEP 4: if(s.charAt(i) != t.charAt(i)) then RETURN s.substring(0, i)
  • STEP 5: i= i+1
  • STEP 6: RETURN s.substring(0,n)
  • STEP 7: END
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All Answers

robaa
2022-11-12
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Program:

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

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