LeetCode – Reverse Words in a String (Java)

Given an input string, reverse the string word by word.

For example, given s = "the sky is blue", return "blue is sky the".

Java Solution

This problem is pretty straightforward. We first split the string to words array, and then iterate through the array and add each element to a new string. Note: StringBuilder should be used to avoid creating too many Strings. If the string is very long, using String is not scalable since String is immutable and too many objects will be created and garbage collected.

class Solution {
	public String reverseWords(String s) {
		if (s == null || s.length() == 0) {
			return "";
		}
 
		// split to words by space
		String[] arr = s.split(" ");
		StringBuilder sb = new StringBuilder();
		for (int i = arr.length - 1; i >= 0; --i) {
			if (!arr[i].equals("")) {
				sb.append(arr[i]).append(" ");
			}
		}
		return sb.length() == 0 ? "" : sb.substring(0, sb.length() - 1);
	}
}

Category >> Algorithms

If you want someone to read your code, please put the code inside <pre><code> and </code></pre> tags. For example:

<pre><code> 
String foo = "bar";
</code></pre>

Berns Dela Vega

val mySentence = “the sky is blue”.split(” “).reverse
mySentence: Array[String] = Array(blue, is, sky, the)

scala> mySentence.mkString(” “)
res24: String = blue is sky the
R M

public static String reverseWords(String s) { Stack stck = new Stack(); if (s == null || s.length() == 0) { return ""; }
// split to words by space String[] arr = s.split(" "); StringBuilder sb = new StringBuilder(); for (int i = arr.length - 1; i >= 0; i--) { //if (!arr[i].equals("") ) { // fails if there are extra spaces between words if( i != 0)// don't append if it is the last word sb.append(arr[i]).append(" "); else sb.append(arr[i]); //} } //return sb.length() == 0 ? "" : sb.substring(0, sb.length() - 1); return sb.toString(); }
R M

prints extra space at the end.
Anuj Kumar

Here’s the most simple way to achieve it:

public class testString {

public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner s=new Scanner(System.in);

System.out.println(“Enter any String:”);

String ss=s.nextLine();

String sb[]= ss.split(” “);
for(int i=sb.length-1;i>=0;i–)
System.out.println(sb[i]);
}

}

Input: Hello World
OutPut: World Hello
STM

Can someone tell me what will be the complexity of this?
melad

using stack
Melad

public static void main(String[] args) { String str = "my name is melad"; Stack stack = new Stack();
String[] strArray = str.split(" "); for (String s : strArray) { stack.push(s); } StringBuilder sb = new StringBuilder(); while (!stack.empty()) { sb.append(stack.pop()).append(" "); }
System.out.println(sb); }
Onkar

private static void reverse_words(String input) {

int i;
System.out.println(“Input is “+input);
StringBuilder sb = new StringBuilder();
String[] temp = input.split(” “);

for (i = temp.length – 1 ; i>= 0; i–)
{
sb.append(temp[i]);
sb.append(” “);
}
System.out.println(“Reversed string is “+sb);
}
Renzo

Only one line in Python: https://github.com/renzon/code_interview_training/blob/master/reverse_words.py
Dolly Agarwal

public class Testcase {

public static void main(String[] args) {

String t = “my name is dolly”;

String c[] = t.split(” “);

//System.out.println(c.length);

//System.out.println(c[0] + c[1] + c[2]);

for(int i =0;i=1;j–){

System.out.print(c[i].charAt(j-1));

}

System.out.print(” “);

}

}

}
Bhavani

String str=”this is a line”;
String str1[]=str.split(” “);
String finalstr=” “;
for(int i=str1.length-1;i>=0;–i)
{
finalstr+=str1[i]+” “;
}
System.out.prntln(finalstr);
sonia

Hi Guys , here in above solution they are using string builder class …what if we are asked to print without using any additional java class object…below is the simple solution :-

public void word_rev(String str)

{

if(str.length()==0)

return ;

int k = str.length()-1 ;

int n = k ;

while(n>=0)

{

while(str.charAt(k)!=’ ‘)

{

if(k==0)

break ;

k– ;

}

if(k==0)

{

System.out.print(str.substring(k , n+1));

break ;

}

else

{

System.out.print(str.substring(k+1 , n+1));

System.out.print(” “);

}

n = k– ;

}

}
Sushant Raj

before reverse:this is band
after reverse:dnab si siht
daksh

public static void main(String[] args) {

String s=”Hello world how’s it going”;

String[] s1=s.split(” “);

StringBuffer reverse=new StringBuffer(” “);

for(int i=(s1.length)-1;i>=0;i–){

reverse.append(s1[i]);

reverse.append(” “);

}

System.out.println(reverse);

}

much easier
Taoufik Bdiri

Here a solution using trim() and replaceAll()

public String reverseWords(String s) {
StringBuilder result = new StringBuilder();
s = s.trim().replaceAll(” +”, ” “);
String[] arr = s.split(” “);
System.out.println(arr.length);
for (int i = 0; i < arr.length; i++) result.append(arr[arr.length – i – 1]).append(" ");
return (result.toString()).substring(0, result.length() – 1);
}
varun

reverseWord(String s){

index = s.indexOf(” “);

return index == -1?s : reverseWord(s.substring(index+1)) + ” “+s.substring(0,index);

}
Jared Burrows

This is not the same.
henrik

And horrible formatting (skills)
Henrik

This is horrible.

private static String reverse(String s)
{
String[] tmp = s.split(” “);
s = “”;
for(int i=0; i<tmp.length; i++)
s = s+tmp[tmp.length-i-1]+" ";
return s.trim();
}
Souraf khan

You give me a very simple ans thanks
Tiago Pinho

This should work to:

public static String reverseString(String input){
StringBuffer result = new StringBuffer();
StringTokenizer tokens = new StringTokenizer(input,” “);

while(tokens.hasMoreElements()){
result.insert(0, tokens.nextToken() + ” “);
}

return result.toString().trim();

}
RK

String in Java is immutable, you know that right?
Steve

Would this solution not work?

public String reverseWordsInStringProblem(String inputData) {
StringBuffer result = new StringBuffer(“”);
String[] parts = inputData.split(” “);
for (int i = parts.length – 1; i >= 0; i–) {
result.append(parts[i]).append(” “);
}
return result.toString();
}
MJCS

public String reverseStringEffective(String input) {

StringBuilder sb= new StringBuilder();

if (input != null || input.length() != 0) {

String[] reverse = input.split(” “);

for (int i = reverse.length – 1; i >= 0; i–) {

sb.append(reverse[i]);

if (i > 0) {

sb.append(” “);

}

}

}return sb.toString();
MJCS

ss = “Hello World”
sb = “dlroW olleH”

Requirement specification of this program:
ss =”Hello World”
sb = “World Hello”
on_fla

One can also use ArrayDeque instead String[], which will much more flexible since it provides FirstAdd(), LastAdd(), LastRemove() and FirstRemove() methods.
kiran

import java.util.*;

class Demo

{

public static void main(String args[])

{

Scanner s=new Scanner(System.in);

System.out.println(“Enter any String:”);

String ss=s.nextLine();

String sb= new StringBuffer(ss).reverse().toString();

System.out.println(“before reverse:” + ss);

System.out.println(“after reverse:” + sb);

}

}
muralikrishna Adusumalli

import java.util.*;

class Demo

{

public static void main(String args[])

{

Scanner s=new Scanner(System.in);

System.out.println(“Enter any String:”);

String ss=s.nextLine();

String sb= new StringBuffer(ss).reverse().toString();

System.out.println(“before reverse:” + ss);

System.out.println(“after reverse:” + sb);

}

}
ramesh

package com.exam;

import java.util.*;

public class MyComparator{

public String comp(String s){

char []ch=s.toCharArray();

StringBuilder sb=new StringBuilder();

for(int i=ch.length-1;i>=0;i–){

sb.append(ch[i]);

}

String m=sb.toString();

return m;

}

public static void main(String args[]){

String s=(“ramesh”);

MyComparator mc=new MyComparator();

String ss=mc.comp(s);

System.out.println(ss);

}

}
Farah El Uasghiri

public static String reverseWords2(String s)
{
String[] tab = s.split(” “);
String temp = “”;
for(int i = 0; i<tab.length/2 ; i++)
{
temp = tab[i];
tab[i] = tab[(tab.length-1)-i];
tab[(tab.length-1)-i] = temp;
}
StringBuffer result = new StringBuffer();
for (int i = 0; i < tab.length; i++)
{
result.append(tab[i]).append(" ");
}
return result.toString();
}
Guest

public static String reverseWords2(String s)
{
String[] tab = s.split(” “);
String temp = “”;
for(int i = 0; i<tab.length/2 ; i++)
{
temp = tab[i];
tab[i] = tab[(tab.length-1)-i];
tab[(tab.length-1)-i] = temp;
}

StringBuffer result = new StringBuffer();
for (int i = 0; i < tab.length; i++)
{
result.append(tab[i]).append(" ");
}
return result.toString();
}
Guest

public static String reverseWords2(String s)
{
String[] tab = s.split(” “);
String temp = “”;
for(int i = 0; i<tab.length/2 ; i++)
{
temp = tab[i];
tab[i] = tab[(tab.length-1)-i];
tab[(tab.length-1)-i] = temp;
}
return Arrays.toString(tab);
}
Cheng

Hi, in your for loop if (!arr[i].equals(“”)) , why you judge “” instead of ” “? Isn’t that it should check if a string in the array contains spaces?
Taras

Wrong, this is required to be done in place – reverse the whole thing, then reverse individual words

LeetCode – Reverse Words in a String (Java)

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