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 (e.g.,"waterbottle"is a rotation of"erbottlewat").

CC150-1.8.java

public class Main {

	/**
	 * 1.8 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 (e.g.,"waterbottle"is
	 * a rotation of"erbottlewat").
	 * 
	 * Solution: just the same as CTCI
	 * If we imagine that s2 is a rotation of s1,then we can
	 * ask what the rotation point is.For example,if you rotate 
	 * waterbottle after wat,you get erbottlewat.In a rotation,we
	 * cut s1 in to two parts,x and y,and rearrange them to get s2.
	 * 
	 * s1=xy=waterbottel
	 * x=wat
	 * y=erbottel
	 * s2=yx=erbottelwat
	 * 
	 * So,we need to check if there's a way to split s1 into x and
	 * y such that xy=s1 and yx=s2.Regardless of where the division 
	 * between x and y is,we can see that yx will always be a
	 * substring of xyxy.That is,s2 will always be a substring of slsl.
	 * 
	 * @Runtime & spaces
	 * runtime: O(n)
	 * space: O(1)
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Main so = new Main();
		String s1="waterbottle";
		String s2="erbottlewat";
		System.out.println(so.isRotation(s1, s2));
	}

	public boolean isRotation(String s1,String s2){
		if(s1==null||s2==null||s1.length()!=s2.length())
			return false;
		if(s1.equals("")&&s2.equals(""))
			return true;
		String s1s1=s1+s1;
		return isSubstring(s1s1,s2);
	}

	private boolean isSubstring(String s1s1, String s2) {
		// TODO Auto-generated method stub
		int len=s2.length();
		for(int i=0;i<len;i++){
			System.out.println(s1s1.subSequence(i, i+len));
			if(s1s1.subSequence(i, i+len).equals(s2))
				return true;			
		}
		return false;
	}
}