Reverse a given sentence in Java

Question

Can anyone tell me how to write a Java program to reverse a given sentence?

For example, if the input is:

"This is an interview question"

The output must be:

"question interview an is this"

Answer 1

You split the string by the space then iterate over it backwards to assemble the reversed sentence.

String[] words =  "This is interview question".split(" ");

String rev = "";
for(int i = words.length - 1; i >= 0 ; i--)
{
   rev += words[i] + " ";
}

// rev = "question interview is This "

// can also use StringBuilder:
StringBuilder revb = new StringBuilder();
for(int i = words.length - 1; i >= 0 ; i--)
{
   revb.append(words[i]);
   revb.append(" ");
}
// revb.toString() = "question interview is This "

Answer 2

String[] words = sentence.split(" ");
String[] reversedWords = ArrayUtils.reverse(words);
String reversedSentence = StringUtils.join(reversedWords, " ");

(using ArrayUtils and StringUtils from commons-lang, but these are easy methods to write - just a few loops)

Answer 3

Just being different: a recursive solution. Doesn't add any extra spaces.

public static String reverse(String s) {
   int k = s.indexOf(" ");
   return k == -1 ? s : reverse(s.substring(k + 1)) + " " + s.substring(0, k);
}


System.out.println("[" + reverse("This is interview question") + "]");
// prints "[question interview is This]"

---

I will also improve on the split solution by using \b instead (it's so obvious!).

    String[] parts = "Word boundary is better than space".split("\\b");
    StringBuilder sb = new StringBuilder();
    for (int i = parts.length; i --> 0 ;) {
        sb.append(parts[i]);
    }
    System.out.println("[" + sb.toString() + "]");
    // prints "[space than better is boundary Word]"

Answer 4

Just split it on a space character into a string array, then loop over the array in reverse order and construct the output string.

String input = "This is interview question";
String output = "";
String[] array = input.split(" ");
for(int i = array.length-1; i >= 0; i--)
{
    output += array[i];
    if (i != 0) { output += " "; }
}

Answer 5

Bozho already gave a great Java-specific answer, but in the event you ever need to solve this problem without Java API methods:

To reverse, you can simply pop individual words onto a stack and pop them all back off when there are no words left.

(Just to be extra clear, Java does provide a Stack class, so it is possible to use this method in Java as well).

Answer 6

a every boring bit of java:

List<String> l = new ArrayList<String>(Arrays.asList("this is an interview question".split("\\s")));
Collections.reverse(l);
StringBuffer b = new StringBuffer();
for( String s : l ){
    b.append(s).append(' ');
}
b.toString().trim();

in groovy it's a little bit more readable:

"this is an interview question"
    .split("\\s")
    .reverse()
    .join(' ')

Answer 7

I also give it a try: Here's a version using a stack and a scanner:

String input = "this is interview question";
Scanner sc = new Scanner(input);
Stack<String> stack = new Stack<String>();

while(sc.hasNext()) {
    stack.push(sc.next());
}

StringBuilder output = new StringBuilder();

for(;;) { // forever
    output.append(stack.pop());

    if(stack.isEmpty()) {
        break; // end loop
    } else {
        output.append(" ");
    }
}

Answer 8

public class ReverseString {

    public void reverse(String[] source) {

        String dest = "";
        for (int n = source.length - 1; n >= 0; n--) {
            dest += source[n] + " ";
        }
        System.out.println(dest);

    }

    public static void main(String args[]) {
        ReverseString rs = new ReverseString();
        String[] str = "What is going on".split(" ");
        rs.reverse(str);

    }

}

Answer 9

I don't think you should use any library.. 1) Reverse whole string 2) Reverse each word.

public static void revWord(char[] a) {

    // reverse whole
    revWord(a, 0, a.length);

    int st = -1;
    int end = -1;

    for (int i = 0; i < a.length; i++) {

        if (st == -1 && a[i] != ' ') {
            st = i;
        }
        if (end == -1 && a[i] == ' ' ) {
            end = i;
        }
        if(i == a.length-1){
            end=i+1;
        }

        if (st != -1 && end != -1) {
            revWord(a, st, end );

            st = -1;
            end = -1;
        }

    }

}

public static void revWord(char[] a, int s, int l) {
    int mid = (l - s) / 2;
    l--;

    for (int i = 0; i < mid; i++, l--) {
        char t = a[s+i];
        a[s+i] = a[l];
        a[l] = t;
    }
}

`

Answer 10

No one has mentioned a vanilla Java 8 based solution yet, which is the same as Bozho's, but without any third-party libraries. So here it is:

String input = "This is interview question";

List<String> list = Arrays.asList(input.split(" "));
Collections.reverse(list);
System.out.println(list.stream().collect(Collectors.joining(" ")));

Answer 11

please try below solution, this is working for me.

public class reverseline {

public static void main(String[] args) {
    // TODO Auto-generated method stub

        String str="This is interview question";
    String words[]=str.split(" ");
    for(int i=words.length-1;i>=0;i--){
        System.out.print(words[i]+" ");
    }
}

}

Answer 12

nicer approach probably.. had seen the logic somewhere..here is my code which might do the job.

    public class revWords {

    public static void main(String[] args) {

        revWords obj = new revWords();
        String print = obj.reverseWords("I am God");
        System.out.println(print);

    }

    public String reverseWords(String words)
    {
      if(words == null || words.isEmpty() || !words.contains(" "))
        return words;

      String reversed = "";
      for( String word : words.split(" "))
        reversed = word + " " + reversed;

      return reversed;
    }

}

Answer 13

Before StringTokenizer was declared legacy, many used StringTokenizer for this. Thought I would just leave it here.

String sentence = "This is interview question";
String reversed = "";
StringTokenizer tokens = new StringTokenizer(sentence); 

while (tokens.hasMoreTokens()) { // Loop through each token
    reversed =  tokens.nextToken() + ' ' + reversed; //add to start
}

System.out.println(reversed.trim());

Answer 14

Shortest Answer

public class ReverseSentence {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter a sentence");
        String inputString = sc.nextLine();

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

        List<String> reverseWord = Arrays.asList(words);
        Collections.reverse(reverseWord);

        Iterator itr = reverseWord.iterator();

        while (itr.hasNext()) {
            System.out.print(itr.next() + " ");
        }
    }

}

OR

public class ReverseSentence {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter a sentence");
        String inputString = sc.nextLine();

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

        for (int i = words.length-1 ; i >= 0; i--) {
            System.out.print(words[i] +" ");
        }
    }

}