Reverse the ordering of words in a string

Question

I have this string s1 = "My name is X Y Z" and I want to reverse the order of the words so that s1 = "Z Y X is name My".

I can do it using an additional array. I thought hard but is it possible to do it inplace (without using additional data structures) and with the time complexity being O(n)?

Answer 1

public class manip{

public static char[] rev(char[] a,int left,int right) {
    char temp;
    for (int i=0;i<(right - left)/2;i++)    {
        temp = a[i + left];
        a[i + left] = a[right -i -1];
        a[right -i -1] = temp;
    }

    return a;
}
public static void main(String[] args) throws IOException {

    String s= "i think this works";
    char[] str = s.toCharArray();       
    int i=0;
    rev(str,i,s.length());
    int j=0;
    while(j < str.length) {
        if (str[j] != ' ' && j != str.length -1) {
            j++;
        } else
        {
            if (j == (str.length -1))   {
                j++;
            }
            rev(str,i,j);
            i=j+1;
            j=i;
        }
    }
    System.out.println(str);
}

Answer 2

Here is the Java Implementation:

public static String reverseAllWords(String given_string)
{
    if(given_string == null || given_string.isBlank())
        return given_string;

    char[] str = given_string.toCharArray();
    int start = 0;

    // Reverse the entire string
    reverseString(str, start, given_string.length() - 1);

    // Reverse the letters of each individual word
    for(int end = 0; end <= given_string.length(); end++)
    {
        if(end == given_string.length() || str[end] == ' ')
        {
            reverseString(str, start, end-1);
            start = end + 1;
        }
    }
    return new String(str);
}

// In-place reverse string method
public static void reverseString(char[] str, int start, int end)
{
    while(start < end)
    {
        char temp = str[start];
        str[start++] = str[end];
        str[end--] = temp;
    }
}

Answer 3

This is assuming all words are separated by spaces:

#include <stdio.h>
#include <string.h>

int main()
{
    char string[] = "What are you looking at";
    int i, n = strlen(string);

    int tail = n-1;
    for(i=n-1;i>=0;i--)
    {
        if(string[i] == ' ' || i == 0)
        {
            int cursor = (i==0? i: i+1);
            while(cursor <= tail)
                printf("%c", string[cursor++]);
            printf(" ");
            tail = i-1;
        }
    }
    return 0;
}

Answer 4

In Smalltalk:

'These pretzels are making me thirsty' subStrings reduce: [:a :b| b, ' ', a]

I know noone cares about Smalltalk, but it's so beautiful to me.

Answer 5

In Python, if you can't use [::-1] or reversed(), here is the simple way:

def reverse(text):

  r_text = text.split(" ")
  res = []
  for word in range(len(r_text) - 1, -1, -1): 
    res.append(r_text[word])

  return " ".join(res)

print (reverse("Hello World"))

>> World Hello
[Finished in 0.1s]

Answer 6

import java.util.Scanner;

public class revString {
   static char[] str;

   public static void main(String[] args) {
    //Initialize string
    //str = new char[] { 'h', 'e', 'l', 'l', 'o', ' ', 'a', ' ', 'w', 'o',
    //'r', 'l', 'd' };
    getInput();

    // reverse entire string
    reverse(0, str.length - 1);

    // reverse the words (delimeted by space) back to normal
    int i = 0, j = 0;
    while (j < str.length) {

        if (str[j] == ' ' || j == str.length - 1) {

            int m = i;
            int n;

            //dont include space in the swap. 
            //(special case is end of line)
            if (j == str.length - 1)
                n = j;
            else
                n = j -1;


            //reuse reverse
            reverse(m, n);

            i = j + 1;

        }
        j++;
    }

    displayArray();
}

private static void reverse(int i, int j) {

    while (i < j) {

        char temp;
        temp = str[i];
        str[i] = str[j];
        str[j] = temp;

        i++;
        j--;
    }
}
private static void getInput() {
    System.out.print("Enter string to reverse: ");
    Scanner scan = new Scanner(System.in);
    str = scan.nextLine().trim().toCharArray(); 
}

private static void displayArray() {
    //Print the array
    for (int i = 0; i < str.length; i++) {
        System.out.print(str[i]);
    }
}

}