JAVA - Reverse String without temp String, array, Stringbuilder, substring

Question

I have a question for you. So, I have to reverse a String with this requirement :

Only use : 1 variable string (for input), 1 variable char & 1 variable int
Can't use another string, string builder, list, array or collections
Output result assigned in input variable
Length of input variable can't be increase
Can't use function substring

And then I try this, is this correct?

  String str = "Hello World";  

  System.out.println("Before : "+str);

  for(int i=0;i<str.length();i++){

   str = new String(str.getBytes(), 1, str.length()-1-i) + new String(str.getBytes(), 0, 1) + new String(str.getBytes(), str.length()-i, i);

   System.out.println(str);

  }

  System.out.println("After : "+str);

Output :

Before : Hello World

ello WorldH

llo WorldeH

lo WorldleH

o WorldlleH

WorldolleH

World olleH

orldW olleH

rldoW olleH

ldroW olleH

dlroW olleH

After : dlroW olleH

using 1 variable means not using additional memory.. cant use new String!.. correct me if wrong.. — karthik manchala, Mar 14 '15 at 15:02

score 2 · Answer 1 · answered Mar 14 '15 at 15:14

2

You can use recursion to the task, something like this:

public static String reverse(String source, int from) {
    if (source.length()-from == 1) {
        return source.charAt(from)+"";
    }
    return reverse(source, from+1) + source.charAt(from);
}

answered Mar 14 '15 at 15:14

Vladislav Lezhnin

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This solution is using two strings reverse and source. – IndieTech Solutions Mar 14 '15 at 15:22
no, if I assign the result to the source variable I will have only one String variable. – Vladislav Lezhnin Mar 14 '15 at 15:25
Check his requirement: Only use : 1 variable string (for input), 1 variable char & 1 variable int – IndieTech Solutions Mar 14 '15 at 15:25
Yes, and this solution completely fulfills those requirements. Where do you see two String variables I do not underestand. – Vladislav Lezhnin Mar 14 '15 at 15:27
String source String reverse are those not two different strings ?!! – IndieTech Solutions Mar 14 '15 at 15:28
`source.charAt(from)+""` is compiled to `new StringBuilder(source.charAt(from).toString()).append("")` – dognose Mar 14 '15 at 15:29
his proposed solution doesnt fulfill the requirement as well that's why he posted the question. check @karthikmanchala comment – IndieTech Solutions Mar 14 '15 at 15:32
I'm not using StringBuilder directly, so does it really matter in this case to which construction it will be compiled? – Vladislav Lezhnin Mar 14 '15 at 15:32
I am really not sure why you're arguing this: it's very clear: Only use : 1 variable string (for input) Can't use another string – IndieTech Solutions Mar 14 '15 at 15:34
@Alundrathedreamwalker he can't use another variable of type String, but he **has** to be allowed to use other String instances. Otherwise, how could he even get the result and assign it to the input? Strings are immutable. – JB Nizet Mar 14 '15 at 15:47
That's my point , fulfilling his requirements aren't possible in JAVA (As far as i know) so even the solution he proposed is wrong. There's no way he can use the same string in Java for both transactions. – IndieTech Solutions Mar 14 '15 at 15:50
@Alundrathedreamwalker I agree with you. I think this is something like an academic task, where solution might seem right, but also it brings a lot to discussion about the language implementation details. – Vladislav Lezhnin Mar 14 '15 at 15:54
reverse a String in JAVA in place is Impossible giving his requirements . other than that there are a lot of ways to achieve it. – IndieTech Solutions Mar 14 '15 at 15:56

IndieTech Solutions · Answer 2 · 2015-03-14T15:42:22.743

1

String is an immutable class in java any methods which seem to modify it always return a new string object with modification. So the answer is, NO you cannot reverse a String in-place in Java. Java String's are implemented as wrappers around a char array, which is hidden from you (i.e. you can only get copies of this array via normal means).

edited Mar 14 '15 at 15:42

answered Mar 14 '15 at 15:22

IndieTech Solutions

2,527
1
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At some level, @VladLezhnin's answer seems right.. because we can use 1 variable char at our disposal.. and he did the same thing by using `charAt()`. – karthik manchala Mar 15 '15 at 04:39

dognose · Answer 3 · 2015-03-14T16:00:36.770

Only use : 1 variable string (for input), 1 variable char & 1 variable int

This implies, that the desired solution is:

Take character from position n from String, store to character variable.
Move variable m to n (in string).
restore cached char from character variable to position m.
repeat until string is fully processed.

This is a typical question asked for "basic" programming languages. However Java does not allow for this, as it leaks the option to set a character based on position within a string.

While this is a no brainer in other languages, java has no option to set values based on an index for Strings.

public static String reverse(String str){
        char c;
        int i = 0;

        for (i=0; i< str.length() / 2; i++){
            c = str.charAt(i);

            //this would be required to match your requirements.
            str[i] = str.charAt(str.length() -1 -i);    
            str[str.length() -1 -i] = c;            
        }

        return str;
    }

but in java, you can only do:

public static String reverse(String str){
    char c;
    int i = 0;

    for (i=0; i< str.length() / 2; i++){
        c = str.charAt(i);
        char[] temp = str.toCharArray();
        temp[i] = str.charAt(str.length() -1-i);
        temp[str.length() -1 -i] = c;
        str = new String(temp);
    }

    return str;
}

which creates additional char arrays and new String objects... Even if you could lower this to one additional char array by declaring it out of the for- loop, it does no longer match the requirement.

I think the guy designing the question was thinking in "C" or "php", where index-based access on strings is possible.

If a String would equal a char-array (which it does in some older languages, that might be the origin of this exercise) it would look like this:

public static char[] reverse(char[] str){
    char c;
    int i = 0;

    for (i=0; i< str.length / 2; i++){
        c = str[i];
        str[i] = str[str.length -1-i];
        str[str.length -1 -i] = c;
    }

    return str;
}

I_Tech_Avi · Answer 4 · 2016-09-26T11:56:29.207

Without using any collection,StringBulider, StringBuffer or temp array reverse the string. Simple and crisp:

public static void main(String[] args) {

    String test = "Hello World";
    String rev = "";
    Pattern p = Pattern.compile("[\\w|\\W]");
    Matcher m = p.matcher(test);
    while (m.find()) {
        rev = m.group()+rev;
    }
    System.out.println("Reverse==" + rev);
}

Output

Reverse==dlroW olleH

Hope it helps :)

JAVA - Reverse String without temp String, array, Stringbuilder, substring

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