Checking if strings are rotations of each other or not

How to check if strings are rotations of each other or not?​

• Both the Strings have equal lengths and consist of same characters.
  
• We can obtain the second string by rotating the first String around a certain character.
  

For example, Suppose we have two strings S1 = "HELLO", and S2 = "LOHEL". Both of them have equal lengths and have the same characters. By rotating "HELLO" three position to the left we can obtain "LOHEL". So, these Strings are rotation of each other.

​​Approach: 

In order to solve this problem, we will firstly check if the Strings are of same length. After that we will concatenate the first string with itself, then check whether the second one is present in the concatenated string or not. This is because if you join a String with itself then it actually contains all rotated versions of itself. 

If the second String exists in the concatenated String, the Strings are rotations of each other. In our example, for S1="HELLO",  the concatenated String will be "HELLOHELLO". As we can see, this concatenated string contains the String "LOHEL" and thus, these Strings are rotations of each other.

Algorithm: checkRotation(s1,s2)

Pseudocode:

Code:

#include<iostream>

using namespace std;

bool checkRotation(string s1, string s2) {
        /* Comparing and checking string lengths */
        if (s1.length() != s2.length())
                return false; //returning false if strings are unequal

        string temp = s1 + s1; //storing concatenated string 

        int n = temp.length();
        int m = s2.length();
        
        // checking if s2 is present in temp
        for(int i = 0; i<n-m; i++)
        {
            int flag = 1, j;
            for(j = 0; j < m-1; j++)
            {
                if(s2[j] != temp[i+j])
                {
                    flag = 0;
                    break;
                }
            }
            
            if(flag == 1){
                return true;// return true if s2 is present in temp
            }
        }
        
        return false;
}

/* Driver code */
int main() {
        string a = "HELLO", b = "LOHEL";
        if (checkRotation(a, b))
                cout<<"Given Strings are rotations of each other.";
        else
                cout<<"Given Strings are not rotations of each other.";
        return 0;
}

class StrRotation {
    static boolean checkRotation(String s1, String s2) {
        /* Comparing and checking string lengths */
        if (s1.length() != s2.length())
            return false;

        String temp = s1 + s1; //storing concatenated string

        if (temp.indexOf(s2) != -1) {
            return true; //returning true if 2nd string is present in concatenated string
        } else {
            return false;
        }

    }

    // Driver code 
    public static void main(String[] args) {
        String a = "HELLO";
        String b = "LOHEL";

        if (checkRotation(a, b))
            System.out.println("Given Strings are rotations of each other.");
        else
            System.out.println("Given Strings are not rotations of each other.");
    }
}

def checkRotation(s1, s2): 
    temp = '' 
  
    # Check if lengths of two strings are equal or not 
    if len(s1) != len(s2): 
        return False
  
    # storing concatenated string 
    temp = s1 + s1 
  
    
    if s2 in temp: 
        return True #returning true if 2nd string is present in concatenated string
    else: 
        return False
  
# Driver program to test the above function 
string1 = "HELLO"
string2 = "LOHEL"
  
if checkRotation(string1, string2): 
    print("Given Strings are rotations of each other.")
else: 
    print("Given Strings are not rotations of each other.")

Output:

Complexity Analysis:

In this way we can check if two given Strings are rotations of each other or not.