How to build a string using recursion

Question

The recursive function receives a string and returns a new string so that each couple of chars are the first and the last char of the original. For example:

receives: abcde
returns: aebdc

receives: 1a2b3c
returns: 1ca32b

public class Ex_6 {

    public static void main(String[] args) {
        String str= "1a2b3c";// 1c a3 2b
        System.out.println(insideOut(str));
    }
    
    public static String insideOut(String str) {
        char ch= str.charAt(0);
        String newStr = "";
        if (str.length()==1 || str.length()==0 ) //the base case (a returns a)
            newStr= str;
        else            //I tried to build the new string
            newStr= ch+str.charAt(str.length()-1) + insideOut(str.substring(1, str.length()-1));
        
        return newStr;  //the output was error ("String index out of range: 0")
            
    }

patrinox · Accepted Answer · 2023-01-04 11:40:13Z

1

Here is a solution for your method ->

public static String insideOut(String str) {
    char ch;
    String newStr = "";
    if (str.length() == 1 || str.length() == 0) {
        newStr = str;
    } else {
        ch = str.charAt(0);
        newStr = Character.toString(ch) + str.charAt(str.length() - 1) + insideOut(
                str.substring(1, str.length() - 1));
    }

    return newStr;
}

answered Jan 4, 2023 at 11:40

patrinox

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2 Comments

Mirel Over a year ago

Hi, thank you , it works perfectly, do you think there is a a way to do it without the Character.toString() method?

patrinox Over a year ago

Yes. You can remove char variable from the method totally and change the line in else condition like this -> newStr = "" + str.charAt(0) + str.charAt(str.length() - 1) + insideOut(str.substring(1, str.length() - 1));

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