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

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.

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

Time Complexity: O(n*n), where n is the length of the string.
Auxiliary Space: O(n)

Library Functions Used:
strstr: 
strstr finds a sub-string within a string. 
Prototype: char * strstr(const char *s1, const char *s2); 
strcat: 
strncat concatenate two strings 
Prototype: char *strcat(char *dest, const char *src); 

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!