A Program to check if strings are rotations of each other or not

2.6

Given a string s1 and a string s2, write a snippet to say whether s2 is a rotation of s1 using only one call to strstr routine?
(eg given s1 = ABCD and s2 = CDAB, return true, given s1 = ABCD, and s2 = ACBD , return false)


Algorithm:
areRotations(str1, str2)

    1. Create a temp string and store concatenation of str1 to
       str1 in temp.
                          temp = str1.str1
    2. If str2 is a substring of temp then str1 and str2 are 
       rotations of each other.

    Example:                 
                     str1 = "ABACD"
                     str2 = "CDABA"

     temp = str1.str1 = "ABACDABACD"
     Since str2 is a substring of temp, str1 and str2 are 
     rotations of each other.

Implementation:

C

   
// C program to check if two given strings are rotations of 
// each other
# include <stdio.h>
# include <string.h>
# include <stdlib.h>

/* Function checks if passed strings (str1 and str2)
   are rotations of each other */
int areRotations(char *str1, char *str2)
{
  int size1   = strlen(str1);
  int size2   = strlen(str2);
  char *temp;
  void *ptr;

  /* Check if sizes of two strings are same */
  if (size1 != size2)
     return 0;

  /* Create a temp string with value str1.str1 */
  temp   = (char *)malloc(sizeof(char)*(size1*2 + 1));
  temp[0] = '\0';
  strcat(temp, str1);
  strcat(temp, str1);

  /* Now check if str2 is a substring of temp */
  ptr = strstr(temp, str2);

  free(temp); // Free dynamically allocated memory

  /* strstr returns NULL if the second string is NOT a
    substring of first string */
  if (ptr != NULL)
    return 1;
  else
    return 0;
}

/* Driver program to test areRotations */
int main()
{
    char *str1 = "AACD";
    char *str2 = "ACDA";

    if (areRotations(str1, str2))
       printf("Strings are rotations of each other");
    else
       printf("Strings are not rotations of each other");

    getchar();
    return 0;
}

Python

   
# Python program to check if strings are rotations of
# each other or not

# Function checks if passed strings (str1 and str2)
# are rotations of each other
def areRotations(string1, string2):
    size1 = len(string1)
    size2 = len(string2)
    temp = ''

    # Check if sizes of two strings are same
    if size1 != size2:
        return 0

    # Create a temp string with value str1.str1
    temp = string1 + string1

    # Now check if str2 is a substring of temp
    # string.count returns the number of occurences of
    # the second string in temp
    if (temp.count(string2)> 0):
        return 1
    else:
        return 0

# Driver program to test the above function
string1 = "AACD"
string2 = "ACDA"

if areRotations(string1, string2):
    print "Strings are rotations of each other"
else:
    print "Strings are not rotations of each other"

# This code is contributed by Bhavya Jain

Java

  
// Java program to check if two given strings are rotations of 
// each other

class StringRotation
{
    /* Function checks if passed strings (str1 and str2)
       are rotations of each other */
    static boolean areRotations(String str1, String str2)
    {
        // There lengths must be same and str2 must be 
        // a substring of str1 concatenated with str1.  
        return (str1.length() == str2.length()) &&
               ((str1 + str1).indexOf(str2) != -1);
    }
    
    // Driver method
    public static void main (String[] args)
    {
        String str1 = "AACD";
        String str2 = "ACDA";

        if (areRotations(str1, str2))
            System.out.println("Strings are rotations of each other");
        else
            System.out.printf("Strings are not rotations of each other");
    }
}
// This code is contributed by  munjal 


Output:
Strings are rotations of each other

Library Functions Used:
strstr:
strstr finds a sub-string within a string.
Prototype: char * strstr(const char *s1, const char *s2);
See http://www.lix.polytechnique.fr/Labo/Leo.Liberti/public/computing/prog/c/C/MAN/strstr.htm for more details

strcat:
strncat concatenate two strings
Prototype: char *strcat(char *dest, const char *src);
See http://www.lix.polytechnique.fr/Labo/Leo.Liberti/public/computing/prog/c/C/MAN/strcat.htm for more details

Time Complexity: Time complexity of this problem depends on the implementation of strstr function.
If implementation of strstr is done using KMP matcher then complexity of the above program is (-)(n1 + n2) where n1 and n2 are lengths of strings. KMP matcher takes (-)(n) time to find a substrng in a string of length n where length of substring is assumed to be smaller than the string.

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