Recursion Doubt with String in Java

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Recursion Doubt with String in Java

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#1 by (2 votes)

2

I'm trying to solve the following exercise:

An S collection of strings is defined recursively by:


'a' and 'b' belong to S;

If X belongs to S, then Xb also belongs to S;


Write a procedure that implements this definition recursively and say which of the following strings belong to S:

(a) to

(b) ab

(c) flap

(d) aaab

(e) bbbbb

My code looks like this:

    static boolean funcao(String str){
        if(str.equals("a") || str.equals("b"))
             return true;
        return(...)

In the last return is where I can not solve, I know that in theory it should be something like this:

   retorna(str[ultimo caracter] == 'b' && funcao(str[do primeiro ao penúltimo caractere]);

But I can not, could anyone help me?

java string

asked by anonymous 25.08.2018 / 17:33

1 answer

List column How to invert control list of Control.ControlCollection?

score 2 · Accepted Answer

str[ultimo caracter]

str.charAt(str.length() - 1)

str[do primeiro ao penúltimo caractere]

str.substring(0, str.length() - 1)

But it still lacks a condition to work because you have to make sure that there is at least one character in the string. This is possible with a if (str.isEmpty()) return false; . You can also take advantage of this if and check if the string is null .

The code looks like this:

private static boolean funcao(String str) {
    if (str == null || str.isEmpty()) return false;
    if (str.equals("a") || str.equals("b")) return true;
    return str.charAt(str.length() - 1) == 'b'
            && funcao(str.substring(0, str.length() - 1));
}