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

• Last Updated : 13 Jan, 2022

Given a string s1 and a string s2, write a snippet to say whether s2 is a rotation of s1?
(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.
```

## C

 `    ` `// C program to check if two given strings are rotations of ``// each other``# include ``# include ``# include `` ` `/* 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 = ``''``;``  ``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;``}`

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 substring in a string of length n where length of substring is assumed to be smaller than the string.
Please refer complete article on A Program to check if strings are rotations of each other or not for more details!

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