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Reverse words in a given String in Java

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Let’s see an approach to reverse words of a given String in Java without using any of the String library function Examples:

Input : "Welcome to geeksforgeeks"
Output : "geeksforgeeks to Welcome"

Input : "I love Java Programming"
Output :"Programming Java love I"

Prerequisite: Regular Expression in Java 

Java

// Java Program to reverse a String
// without using inbuilt String function
import java.util.regex.Pattern;
public class Exp {
 
    // Method to reverse words of a String
    static String reverseWords(String str)
    {
 
        // Specifying the pattern to be searched
        Pattern pattern = Pattern.compile("\\s");
 
        // splitting String str with a pattern
        // (i.e )splitting the string whenever their
        // is whitespace and store in temp array.
        String[] temp = pattern.split(str);
        String result = "";
 
        // Iterate over the temp array and store
        // the string in reverse order.
        for (int i = 0; i < temp.length; i++) {
            if (i == temp.length - 1)
                result = temp[i] + result;
            else
                result = " " + temp[i] + result;
        }
        return result;
    }
 
    // Driver methods to test above method
    public static void main(String[] args)
    {
        String s1 = "Welcome to geeksforgeeks";
        System.out.println(reverseWords(s1));
 
        String s2 = "I love Java Programming";
        System.out.println(reverseWords(s2));
    }
}

                    

Output:

geeksforgeeks to Welcome
Programming Java love I

Time Complexity: O(n), where n is the length of the string.

Auxiliary Space: O(n)

Approach: Without using split() or trim()

By this approach, we can even remove extra trailing spaces and in between the words also.

Basically, this algorithm involves 3 steps.

  1. If you find white space, there can be two possibilities. 
  2. It might be end of a word or else extra trailing space in between the words.
  3. if it is not a white space, add the character to temporary word as shown in the below code.

Below is the implementation of above approach.

Java

import java.util.*;
 
class GFG {
     
    public static String reverseString(String s)
    {
        StringBuilder ans=new StringBuilder();
         
        String temp = "";
        for(int i=0;i<s.length();i++)
        {
            char ch = s.charAt(i);
            if(ch==' ')
            {
                //if we find white space add temp in the start
                if(!temp.equals(""))
                {
                    //adding in the front every time
                    ans.insert(0,temp+" ");
                }
                 
                temp = "";
            }
            else
                temp += ch;
        }
       
        //just removing the extra space at the end of the ans
        return ans.toString().substring(0,ans.length()-1);
         
    }
     
    public static void main(String[] args) {
         
        String s1="   Welcome to   Geeks For Geeks   ";
        System.out.println("Before reversing length of string : "+s1.length());
        String ans1=reverseString(s1);
        System.out.println("After reversing length of string : "+ans1.length());
        System.out.println("\""+ans1+"\"\n");
         
        String s2="   I Love Java   Programming      ";
        System.out.println("Before reversing length of string : "+s2.length());
        String ans2=reverseString(s2);
        System.out.println("After reversing length of string : "+ans2.length());
        System.out.println("\""+ans2+"\"");
         
    }
}
//This code is contributed by aeroabrar_31

                    

Output
Before reversing length of string : 34
After reversing length of string : 26
"Geeks For Geeks to Welcome"

Before reversing length of string : 34
After reversing length of string : 23
"Programming Java Love I"

Time Complexity: O(N) N is length of string

Auxiliary Space: O(1) 

You can find the c++ solution for Reverse words in a String here


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Last Updated : 29 Mar, 2023
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