CheckAB.java

// Check AB
// Send Feedback
// Suppose you have a string, S, made up of only 'a's and 'b's. Write a recursive function that checks if the string was generated using the following rules:
// a. The string begins with an 'a'
// b. Each 'a' is followed by nothing or an 'a' or "bb"
// c. Each "bb" is followed by nothing or an 'a'
// If all the rules are followed by the given string, return true otherwise return false.
// Input format :
// String S
// Output format :
// 'true' or 'false'
// Constraints :
// 1 <= |S| <= 1000
// where |S| represents length of string S.
// Sample Input 1 :
// abb
// Sample Output 1 :
// true
// Sample Input 2 :
// abababa
// Sample Output 2 :
// false
// Explanation for Sample Input 2
// In the above example, a is not followed by either "a" or "bb", instead it's followed by "b" which results in false to be returned.

import java.util.Scanner;

public class CheckAB {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        System.out.print("Enter String : ");
        String input = s.nextLine();
        boolean ans = checkAB(input);
        System.out.println("Output : " + ans);
        s.close();
    }

    public static boolean checkAB(String input)
    {
        if(input.length()==0)
        {
            return true;
        }
        if(input.charAt(0)!='a')
        {
            return false;
        }
        if(input.length()>=3 && "abb".equals(input.substring(0,3))){
            return checkAB(input.substring(3));
        }
        else
        {
            return checkAB(input.substring(1));
        }
        
    }
}