CareerCup

Here is my code in java (take from k2code.blogspot.in/2011/12/how-will-you-check-if-s1-is-rotated.html) :

boolean isRotation(String s1,String s2) {
    return (s1.length() == s2.length()) && ((s1+s1).indexOf(s2) != -1);
}

//Assume you have a method isSubstring which checks if one word is a substring of another.
    //	Given two strings, s1 and s2, write code to check if s2 is a rotation of s1 using only one call to isSubstring i.e., waterbottle is a rotation of erbottlewat
	// COMPLEXITY O ( n^2 ) + O (n + m) = O ( n^2 )
	public static boolean isRotation(char str1 [], char str2[] )
	{
		return isSubstring(append(str2,str2),str1);
	}
	// COMPLEXITY  O ( n + m )
	public static char[] append(char str1[], char str2[]){
		char result[] = new char[str1.length + str2.length];
		int index = 0;
		int len1 = str1.length; 
		while(index < str1.length)
			result[index] = str1[index++];
		index = 0;
		while(index < str2.length)
			result[len1 + index] = str2[index++];
		
		return result;
	}
	// COMPLEXITY O ( N^2 )
	public static boolean isSubstring(char str1[], char str2[]){
		int counter = 0;
		for(int i = 0; i < str2.length; i++){
			int start = i;
			while(counter < str2.length && start < str1.length && str1[start++] == str2[counter++]){
			    if(counter == str2.length)return true;
			}
			counter = 0;
		}
		return false;
	}

How about this?

isSubstring (s2+s2, s1)

I think this is a concatenation problem.
Concatenate the input string s1 with itself and check if s2 is contained in s1.concat(s1).

public boolean isRoatate(String s1, String s2){
s1+=s1;
return isSubstring (s1,s2);
}

public boolean isRoatate(String s1, String s2){
    	s1+=s1;
    	return isSubstring (s1,s2);

}

You have to check that s1.size() == s2.size() too, otherwise s1 = waterbottle, s2 = water will return true.

public static boolean rotation_check(String str1, String str2){

if(str1.length() != str2.length())
return false;

return (str1+str1).replace(str2,"").equals(str1);

}

Amazing solution, can you please explain why this solution is correct?

public class rotationstring {

public static boolean rotation_check(String str1, String str2){
String temp = str1+str1;

if(str1.length() != str2.length())
return false;
if(temp.toLowerCase().contains(str2.toLowerCase()))
return true;

return false;
}

public static void main(String[] args){
String st1 = "waterbottle";
String st2 = "erbottlewat";
System.out.println(rotation_check(st1, st2));
}

}