I'm trying to understand the below question. I kind of know what the method is doing, and it returns True. Also, I don't understand the question. I believe the answer is 2, but I don't know why. Below is the method and question.
In the ABBAB language the alphabet has only two letters. A string of letters (including and one-letter strings) is a valid word, if and only if the isValid method returns true for that string. isValid is defined as follows:
public boolean isValid(String word)
{
int n = word.length();
return n < 1 ||
(isValid(word.substring(0, n-1) &&
word.charAt(n-1) == 'B') ||
(isValid(word.substring(0, n-2) &&
"BA".equals(word.substring(n-2)));
}
How many valid words of length 7 are there in the ABBABA language?
2 3 15 23 34
(isValid(word.substring(0, n-1) && word.charAt(n-1) == 'B')says a string is valid if the last letter has to be 'B', and the rest of string is also valid (by applying the recursive definition of valid to the rest of the string. This will tell you valid strings are of the form******B– Ben Jul 30 '14 at 22:35