Java API, recursive & iterative algorithm to reverse string (example)

• Given a String in java,  reverse the String using standard java classes & recursive/iterative algorithm.
  • e.g. reverse(“Lived”) = “deviL”
  • reverse(“Star”) = “ratS”
• We will reverse string using following methods.
  • Reverse string using StringBuilder class.
  • Reverse String using StringBuffer class.
  • Java program to reverse String using iterative algorithm.
  • Java program to reverse String using recursive algorithm.

Method 1: Reverse String using StringBuilder class

public static String reverseUsingStringBuilder(String inputString) {
        //inputString = "Lived"
 StringBuilder builder = new StringBuilder(inputString);
        //Output will be: deviL
 return builder.reverse().toString();
}

Method 2: Reverse String using StringBuffer class

public static String reverseUsingStringBuffer(String inputString) {
        //inputString = "Lived"
 StringBuffer builder = new StringBuffer(inputString);
        //Output will be: deviL
 return builder.reverse().toString();
}

Method 3: Java program to reverse string – iterative algorithm

//inputString = "Lived"
public static String iterative(String inputString) {
 if (null == inputString || inputString.length() == 0)
  return "";

 int length = inputString.length();
 char[] chInputArray = inputString.toCharArray();
 for (int index = 0; index < length / 2; index++) {
  char firstHalf = chInputArray[index];
  char secondHalf = chInputArray[length - index - 1];

  //swap
  chInputArray[index] = secondHalf;
  chInputArray[length - index - 1] = firstHalf;
 }
 return new String(chInputArray);
}//Reverse string of "Lived" is "deviL"

Method 4: Java program to reverse string – recursive algorithm

//inputString = "Lived"
public static String recursive(String inputString) {

 if (null == inputString || inputString.length() == 0)
  return "";
 return recursive(inputString.substring(1)) + inputString.charAt(0);
}//Reverse string of "Lived" is "deviL"

Logical flow to reverse string is shown Fig 1.

Complete Program to reverse a string in java:

package org.learn;

import java.util.Scanner;

public class StringReverse {

 public static String reverseUsingStringBuilder(String inputString) {
  StringBuilder builder = new StringBuilder(inputString);
  return builder.reverse().toString();
 }

 public static String reverseUsingStringBuffer(String inputString) {
  StringBuffer builder = new StringBuffer(inputString);
  return builder.reverse().toString();
 }

 public static String iterative(String inputString) {
  
  if (null == inputString || inputString.length() == 0)
   return "";

  int length = inputString.length();
  char[] chInputArray = inputString.toCharArray();
  for (int index = 0; index < length / 2; index++) {
   
   char firstHalf = chInputArray[index];
   char secondHalf = chInputArray[length - index - 1];

   // swap
   chInputArray[index] = secondHalf;
   chInputArray[length - index - 1] = firstHalf;
  }
  return new String(chInputArray);
 }

 public static String recursive(String inputString) {

  if (null == inputString || inputString.length() == 0)
   return "";
  return recursive(inputString.substring(1)) + inputString.charAt(0);
 }

 public static void main(String[] args) {
  try (Scanner scanner = new Scanner(System.in)) {
   System.out.printf("1. Enter input string : ");
   String input = scanner.next();

   String reverse = reverseUsingStringBuilder(input);
   System.out.printf("2. reverseUsingStringBuilder(\"%s\"): %s", input, reverse);

   reverse = reverseUsingStringBuffer(input);
   System.out.printf("\n3. reverseUsingStringBuffer(\"%s\"): %s", input, reverse);

   reverse = iterative(input);
   System.out.printf("\n4. iterative(\"%s\"): %s", input, reverse);

   reverse = recursive(input);
   System.out.printf("\n5. recursive(\"%s\"): %s", input, reverse);
  }
 }
}

Program output to reverse string in java:

1. Enter input string : Lived
2. reverseUsingStringBuilder("Lived"): deviL
3. reverseUsingStringBuffer("Lived"): deviL
4. iterative("Lived"): deviL
5. recursive("Lived"): deviL

1. Enter input string : Star
2. reverseUsingStringBuilder("Star"): ratS
3. reverseUsingStringBuffer("Star"): ratS
4. iterative("Star"): ratS
5. recursive("Star"): ratS