Question about substring and the recursion

Question

Why this return the string reversed, and why this is a recursion function?

public static String Reverse(String a){                                 
        String result = "";
        if(a.length()>0){
            result = Reverse(a.substring(1))+a.charAt(0);
        }
        return result;
    }

@markspace - Yes, this is a recursive function that reverses a String. I'm not sure of the reason behind your comments. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Apr 4, 2020 at 20:20
@Hana I would recommend you step through this with your debugger, to find out how it does what it does. That would be worth 1000 words of explanation. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Apr 4, 2020 at 20:21
@markspace have you tried it? Because from here it looks correct and recursive. — Federico klez Culloca
– Federico klez Culloca, Commented Apr 4, 2020 at 20:21

Beri · Accepted Answer · 2020-04-04 20:30:28Z

3

It can be defined in steps:

Split any string into first character and the 'rest'.
If the 'rest' length is greater than 0, then repeat step 1 for input 'rest'
When the recursion will get to the last character, it will start to print the characters in reverse.

The last recursive execution will return a empty string. To this empty statement it will add character from the previous call (n-1 character), then to this it will add character from position (n-2), and so on. This way we will get a recursive function:

  public static String reverse(String a){   
    if(a.length()>0){
        return reverse(a.substring(1))+a.charAt(0);
    }
    return ""; // last statement
 }

Example:

 'cat' -> reverse('at') + 'c'
          (reverse('t') + 'a') + 'c'
          ((reverse('') + 't') + 'a' + 'c')
          => 'tac'

edited Apr 4, 2020 at 20:30

answered Apr 4, 2020 at 20:26

Beri

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1 Comment

Beri Over a year ago

When a.length<1, then it can only be an empty string. So no difference there.

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