How to reverse words in a String

Reverse words in a String

We can reverse  the words in the String by using StirngBuilder as below:

package com.interviewCodes.String;

public class ReverseWordsInString {

      

       public static void main(String args[]){

              String str="Hi I am John";

              System.out.println("Sentence before reverse:"+str);

              System.out.println("Sentence after reversing:"+reverseWords(str));

       }

      

       public static String reverseWords(String sentence){

              StringBuilder sb=new StringBuilder(sentence.length()+1);

              String[] words=sentence.split(" ");

              for(int i=words.length-1;i>=0;i--){

                     sb.append(words[i]).append(' ');

              }

              sb.setLength(sb.length()-1);

              return sb.toString();

       }

}

This is the preferred way to reverse the words in a  String.Please comment the other ways of reversing the words in a String.Thanks for reading.

How to reverse a number in Java

Reverse a number/Integer

We can reverse a number by converting it into a String or iteratively or recursively.

Now we will how to reverse  number by converting it into a String.

package com.interviewCodes.String;

public class ReverseNumberDemo1 {

      

       public static void main(String[] args) {

              int num=12456;

              //converting Integer into a String

              String str=String.valueOf(num);

              //Now we can reverse the String as shown in my previous post

              //And then convert that String into Integer like below

              Integer revNum=Integer.parseInt(str);

       }

}

We can reverse the Number by iteratively or recursively as below:

package com.interviewCodes.String;

import java.util.Scanner;

public class ReverseNumberDemo{

       public static void main(String[] args) {

               

         int num, reverseNum = 0;

      System.out.println("Enter the number to reverse");

      Scanner in = new Scanner(System.in);

      num = in.nextInt();

      //reverse a number iteratively

      reverseNum=reverseIteratievly(num);

      System.out.println("Reverse a number Iteratively : "+reverseNum);

      //reverse a number recursively

      reverseNum=reverseRecursievly(num);

      System.out.println("Reverse a number Iteratively : "+reverseNum);

    }

       public static int reverseIteratievly(int num) {

              int reverse=0;

           while( num != 0 )

             {

                 reverse = reverse * 10;

                 reverse = reverse + num%10;

                 num = num/10;

             }

             return reverse;

       }

    public static int  reverseRecursievly(int num) {

             

       return _reverseRecurievly(num,0);

     }

     public static int _reverseRecurievly(int n,int r){

       if(n==0){

              return r;

       }

       else{

              return _reverseRecurievly(n/10,r*10+n%10);

       }

}

}

That's all about reverse a number.If any other ways are there to reverse a number,please comment.Thanks for reading.

How to reverse String in Java

Reverse String in Java

Hi this is the frequent question that I faced in some of the interviews. So I choose this one to write as my first post.

We can reverse a String using StringBuffer or StringBuilder or without using these two by iteratively or recursively.

First we will see how to reverse a String using StringBuffer and StringBuilder reverse() method.

package com.interviewCodes.String;

public class ReverseStringDemo1 {

       public static void main(String[] args) {

      String orgStr = "Hello World";

      System.out.println("Original String : " + orgStr );

       // reverse a String using StringBuffer

       String revStr = new StringBuffer(orgStr).reverse().toString();

       System.out.println("Reversed String "+ revStr);

       // reverse a String using StringBuilder

       revStr = new StringBuilder(orgStr).reverse().toString();

       System.out.println("Reversed String : "+ revStr);

       }

}

Now we  will how to reverse a String without using StringBuffer and StringBuilder by iteratively or recursively.

package com.interviewCodes.String;

public class ReverseStringDemo2 {

       public static void main(String[] args) {

             String orgnStr = "Hello World";

             System.out.println("Original String : " + orgnStr );

             //reverse a String iteratively

             String revStr = reverse(orgnStr);

             System.out.println("Reversed String "+ revStr);

             //reverse a String recursively

             revStr = reverseRecursively(orgnStr);

             System.out.println("Reversed String "+ revStr);

       } 

       // iterative method to reverse a String

       public static String reverse(String str) {

              if (str == null || str.isEmpty()) {

                     return str;

              }

              String reverse = "";

              for (int i = str.length() - 1; i >= 0; i--) {

                     reverse = reverse + str.charAt(i);

              }

              return reverse;

       }

       // reverse a String recursivly

       public static String reverseRecursively(String str) {

              if (str == null || str.isEmpty() || str.length() < 2) {

                     return str;

              }

              return reverseRecursively(str.substring(1)) + str.charAt(0);

       }

}
     

But I prefered to use StringBuffer or StringBuilder libraries to reverse a String.Please suggest if there are another ways of reversing a String.Thanks for reading.