Program to reverse the words in a string

Question

I have written a program to reverse the words in a string. if i/p is "The dog is chasing" then o/p should be "chasing is dog The"

public class String_reverse {

    public static void main(String[] args) {

        String input= "The dog is chasing";

        String[] arr= input.split(" ");
        String reverse="";
        for(int i=arr.length-1;i>=0;i--)
        {
            reverse+= ((reverse.equals(""))?"":" ")+arr[i];
        }
        System.out.println(reverse);

    }

}

But I don't know how to write this program using recursion. When I tried searching in stackoverflow, I could find reversing a string; but not reversing the words in a string.

Answer 1

Recursive method, using same logic as linked "duplicate", without use of split():

private static String reverseWords(String text) {
    int idx = text.indexOf(' ');
    return (idx == -1 ? text : reverseWords(text.substring(idx + 1)) + ' ' + text.substring(0, idx));
}

The logic is:

1. Take first char/word
2. If that is last char/word, return with it
3. Perform recursive call with remaining text (excluding word-separating space).
4. Append space (if doing word)
5. Append first char/word from step 1
6. Return result

As you can see, when applied to reversing text (characters) instead of words, it's very similar:

private static String reverseText(String text) {
    return (text.length() <= 1 ? text : reverseText(text.substring(1)) + text.charAt(0));
}

---

For people who like things spelled out, and dislike the ternary operator, here are the long versions, with extra braces and support for null values:

private static String reverseWords(String text) {
    if (text == null) {
        return null;
    }
    int idx = text.indexOf(' ');
    if (idx == -1) {
        return text;
    }
    return reverseWords(text.substring(idx + 1)) + ' ' + text.substring(0, idx);
}
private static String reverseText(String text) {
    if (text == null || text.length() <= 1) {
        return text;
    }
    return reverseText(text.substring(1)) + text.charAt(0);
}

Notice how the long version of reverseText() is exactly like the version in the linked duplicate.

Answer 2

This is a sample program which will do what you want recursively:

public class ReverseWords {
    public static void main(String args[]) {
        String s = "This is a test";
        reverse(s);
    }

    private static void reverse(String s) {
        if(s == null) return;
        String words[] = s.split(" ", 2);
        if (words.length < 2) reverse(null);
        else reverse(words[1]);
        System.out.print(words[0] + " ");
    }
}

Answer 3

Recursive methods could seem a bit hard when beginning but try to do the following :

• Simplify the problem as much as possible to find yourself with the less complicated case to solve. (Here for example, you could use a sentence with two words).

• Begin with doing it on a paper, use Pseudocode to help you dealing with the problem with the simplest language possible.

• Begin to code and do not forget an escape to your recursion.

---

Solution

public static void main(String[] args) {
    String s = reverseSentence("This sentence will be reversed - I swear".split(" "));
    System.out.println(s);
}

public static String reverseSentence(String[] sentence){
    if (sentence.length <= 1){
        return sentence[0];
    }
    String[] newArray = new String[sentence.length-1];
    for (int i = 0 ; i < newArray.length ; i++){
        newArray[i] = sentence[i];
    }
    return sentence[sentence.length-1] + " " + reverseSentence(newArray);
}

Answer 4

You can refer to the following code.

  class ReverseString {

public static void main(String args[]) {
    String myString = "The dog is chasing";

    reverse(myString);
}

public static String reverse(String myString) {
    int space = myString.indexOf(" ");
    if (space != -1) {
        reverse(myString.substring(space + 1, myString.length()));
    }
    if (space == -1) {
        System.out.println(myString.substring(0, myString.length()));
    } else {
        System.out.println(myString.substring(0, space));
    }
    return myString;
}
}