Question

java/netbeans

• Write a recursive method, reverseString, that accepts a String and returns the String reversed.
• Write a recursive method, reverseArrayList, that accepts an ArrayList of Strings and returns an ArrayList in reserve order of the input ArrayList.
• Write a main method that asks the user for a series of Strings, until the user enters “Done” and puts them in an ArrayList. Main should make use to reverseArrayList and reverseString to reverse each String in the ArrayList and then reverse the ArrayList.

Answer 1

All the explanations is given in the comments of the code itself.

Code--
import java.util.*;
public class Reverse
{
   //method to reverse a string
   public static String reverseStr(String st)
   {
       //base condition for recursion
       if(st.isEmpty())
           return st;
       else
       {
           //remove the first character of the string
           //concat it with the returned string
           //and pass the string again recursively without the
           //first character
           return reverseStr(st.substring(1))+st.charAt(0);
       }
   }
   //method to reverse Arraylist
   public static ArrayList<String> reverseAL(ArrayList<String> al)
   {
       if(al.size()>1)
       {
           //remove the first node of the al
           String temp=al.remove(0);
           //then again call reverseAL
           reverseAL(al);
           //add the temp to al
           al.add(temp);
       }
       //base condition for recursion that is if the al is empty
       return al;
   }
   //main method
   public static void main(String[] args)
   {
       Scanner sc=new Scanner(System.in);
       ArrayList<String> al = new ArrayList<String>();
       String temp;
       int size=0;
       //prompt the user to give inputs until Done is entered
       System.out.println("Enter Inputs strings: ");
       while(true)
       {
           temp=sc.nextLine();
           if(temp.equals("Done"))
           {
               break;
           }
           else
           {
               al.add(temp);
               size++;
           }
       }
       //iterate over the array list and reverse the strings using the above function
       for(int i=0;i<size;i++)
       {
           temp=reverseStr(al.get(i));
           al.set(i, temp);
       }
       //call the function to reverse the arraylist
       al=reverseAL(al);
       //display the output after reversals
       System.out.println("Output");
       for(int i=0;i<size;i++)
       {
           System.out.println(al.get(i));
       }
   }
}

Code Screenshot--

Output Screenshot--

Note--

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