辅助方法：将字符串中的字符加倍，但在第一个字符后停止递归

Question

This is a homework assignment that I got stuck on. I am giving an ArrayList and I need to return a String with all the characters doubled. Input: abc, Output:aabbcc. What I'm getting as my output is aa, then it stops. I've done this multiple times with a loop but I can't seem to get it with recursion.

I tried re-editing my code and also tried using a counter but I either get a null return or I'll get an overstack flow error. I went to Oracles website to figure out other String methods I could use and tried changing my code with other methods but still no luck.

My other methods work of turning the ArrayList to a String but it's my last method I'm having troubles with.

public String dupEachChar2(String str) {
    //str is going to be a string with abc in it.
    String x = "";
    int count = 0;

    // I'm checking if there is only 1 char, if so just return it twice to 1 string.
    if(str.length() == 1) {
        return str + str;
    } else if(count != str.length()) {
        //Doubling part that I can not figure out.
        x += str.substring(count, count+1) + str.substring(count, count+1);
        count++;
        dupEachChar2(str.substring(count));
    }
    return x;
}

Answer 1

You can use the back reference feature of String.replaceAll(). To replace each character of a string with duplicates do the following:

String a = "abc";
String b = a.replaceAll("(.)", "$1$1");
b is now equals to aabbcc

Here is how this works.

• . means match any character and the () are a group capture for the regular expression.
• $1 means get what was matched in the first (ie #1) group. Then simply use it twice to double the character.

Answer 2

try this

public String dupEachChar2(String str) {

        if(str.length() == 1) 
            return str + str;
        else 
            return str.substring(0,1)+str.substring(0,1)+dupEachChar2(str.substring(1));
    }
}

the issue with your code is you're treating the variable x (and count) as if it is shared between every instance of the method, when in fact it is a new variable for every time that method is called. Besides, you don't need the variable x here because you intended for it to hold the entire new string, but you already have that from the value returned by the prior iteration of your recursive method. As for count, you can just look at how many characters are left in the string, (chopping one off every time) and stopping when they reach 1.

If you were to put in the minimal amount of effort to make your code function correctly, I would say pass x in as a second parameter, and use "" when you first call it from wherever.