Java Program To Reverse Words In A Given String

Example: Let the input string be “i like this program very much”. The function should change the string to “much very program this like i”

Examples: 

Input: s = “geeks quiz practice code” 
Output: s = “code practice quiz geeks”

Input: s = “getting good at coding needs a lot of practice” 
Output: s = “practice of lot a needs coding at good getting”

Algorithm:  

• Initially, reverse the individual words of the given string one by one, for the above example, after reversing individual words the string should be “i ekil siht margorp yrev hcum”.
• Reverse the whole string from start to end to get the desired output “much very program this like i” in the above example.

Below is the implementation of the above approach: 

Java

// Java program to reverse
// a String
import java.util.*;
class GFG{

// Reverse the letters of
// the word
static void reverse(char str[],
                    int start,
                    int end)
{
  // Temporary variable to
  // store character
  char temp;

  while (start <= end)
  {
    // Swapping the first and
    // last character
    temp = str[start];
    str[start] = str[end];
    str[end] = temp;
    start++;
    end--;
  }
}
// Function to reverse words
static char[] reverseWords(char []s)
{
  // Reversing individual words as
  // explained in the first step
  int start = 0;
  for (int end = 0; end < s.length; end++)
  {
    // If we see a space, we reverse the
    // previous word (word between
    // the indexes start and end-1
    // i.e., s[start..end-1]
    if (s[end] == ' ')
    {
      reverse(s, start, end);
      start = end + 1;
    }
  }

  // Reverse the last word
  reverse(s, start, s.length - 1);

  // Reverse the entire String
  reverse(s, 0, s.length - 1);
  return s;
}

// Driver Code
public static void main(String[] args)
{
  String s =
  "i like this program very much ";
  char []p = reverseWords(s.toCharArray());
  System.out.print(p);
}
}
// This code is contributed by gauravrajput1

Output

much very program this like i

Time complexity: O(n)

Auxiliary Space: O(n)

Another Approach:

we can do the above task by splitting and saving the string in a reverse manner. 

Below is the implementation of the above approach:

Java

// Java program to reverse a
// string
public class ReverseWords
{
    public static void main(String[] args)
    {
        String s[] =
        "i like this program very much".split(" ");
        String ans = "";
        for (int i = s.length - 1; i >= 0; i--)
        {
            ans += s[i] + " ";
        }
        System.out.println("Reversed String:");
        System.out.println(ans.substring(0,
                           ans.length() - 1));
    }
}

Output:

Reversed String:
much very program this like i

Time Complexity: O(n) 

Auxiliary Space: O(n) for array s

Without using any extra space:
The above task can also be accomplished by splitting and directly swapping the string starting from the middle. As direct swapping is involved, less space is consumed too.   

Below is the implementation of the above approach:

Java

// Java code to reverse a string
class GFG{

// Reverse the string
public static String[] RevString(String[] s,
                                 int l)
{
    // Check if number of words is even
    if (l % 2 == 0)
    {
        // Find the middle word
        int j = l / 2;

        // Starting from the middle
        // start swapping words at
        // jth position and l - 1 - j position
        while (j <= l - 1)
        {
            String temp;
            temp = s[l - j - 1];
            s[l - j - 1] = s[j];
            s[j] = temp;
            j += 1;
        }
    }

    // Check if number of words is odd
    else
    {
        // Find the middle word
        int j = (l / 2) + 1;

        // Starting from the middle start
        // swapping the words at jth
        // position and l-1-j position
        while (j <= l - 1)
        {
            String temp;
            temp = s[l - j - 1];
            s[l - j - 1] = s[j];
            s[j] = temp;
            j += 1;
        }
    }

    // Return the reversed sentence
    return s;
}

// Driver Code
public static void main(String[] args)
{
    String s = "getting good at coding " +
               "needs a lot of practice";
    String[] words = s.split("\s");
    words = RevString(words, words.length);
    s = String.join(" ", words);
    System.out.println(s);
}
}
// This code is contributed by MuskanKalra1

Output:

practice of lot a needs coding at good getting

Time complexity: O(n)

Auxiliary Space: O(n)

Please refer complete article on Reverse words in a given string for more details!