Reverse a string preserving space positions

Difficulty Level : Easy
Last Updated : 27 Nov, 2020

Write a program to reverse the given string while preserving the position of spaces.

Examples:

Input  : "abc de"
Output : edc ba

Input : "intern at geeks"
Output : skeegt an retni

Input : "Help others"
Output : sreh topleH

Create a string to store results. Mark the space position of the given string in this string.
Insert the character from the input string into the result string in reverse order.
While inserting the character check if the result string already contains a space at index ‘j’ or not. If it contains, we copy the character to the next position.

Below is the implementation of the above steps.

C++

// C++ program to reverse a string preserving
// spaces.
#include <iostream>
using namespace std;
 
// Function to reverse the string
// and preserve the space position
string reverses(string str)
{
    // Mark spaces in result
    int n = str.size();
    string result(n, '\0');
    for (int i = 0; i < n; i++)
        if (str[i] == ' ')
            result[i] = ' ';
 
    // Traverse input string from beginning
    // and put characters in result from end
    int j = n - 1;
    for (int i = 0; i < str.length(); i++) {
        // Ignore spaces in input string
        if (str[i] != ' ') {
            // ignore spaces in result.
            if (result[j] == ' ')
                j--;
 
            result[j] = str[i];
            j--;
        }
    }
 
    return result;
}
 
// Driver code
int main()
{
    string str = "internship at geeks for geeks";
    cout << reverses(str) << endl;
    return 0;
}

Java

// Java program to reverse a string
// preserving spaces.
public class ReverseStringPreserveSpace {
    // Function to reverse the string
    // and preserve the space position
    static void reverses(String str)
    {
 
        char[] inputArray = str.toCharArray();
        char[] result = new char[inputArray.length];
 
        // Mark spaces in result
        for (int i = 0; i < inputArray.length; i++) {
            if (inputArray[i] == ' ') {
                result[i] = ' ';
            }
        }
 
        // Traverse input string from beginning
        // and put characters in result from end
        int j = result.length - 1;
        for (int i = 0; i < inputArray.length; i++) {
 
            // Ignore spaces in input string
            if (inputArray[i] != ' ') {
 
                // ignore spaces in result.
                if (result[j] == ' ') {
                    j--;
                }
                result[j] = inputArray[i];
                j--;
            }
        }
        System.out.println(String.valueOf(result));
    }
 
    // driver function
    public static void main(String[] args)
    {
        reverses("internship at geeks for geeks");
    }
}
 
// This code is contributed by Rishabh Jain

C#

// C# program to reverse a
// string preserving spaces.
using System;
 
class GFG {
 
    // Function to reverse the string
    // and preserve the space position
    static void reverses(string str)
    {
        char[] inputArray = str.ToCharArray();
        char[] result = new char[inputArray.Length];
 
        // Mark spaces in result
        for (int i = 0; i < inputArray.Length; i++) {
            if (inputArray[i] == ' ') {
                result[i] = ' ';
            }
        }
 
        // Traverse input string from beginning
        // and put characters in result from end
        int j = result.Length - 1;
        for (int i = 0; i < inputArray.Length; i++) {
 
            // Ignore spaces in input string
            if (inputArray[i] != ' ') {
 
                // ignore spaces in result.
                if (result[j] == ' ') {
                    j--;
                }
                result[j] = inputArray[i];
                j--;
            }
        }
        for (int i = 0; i < result.Length; i++)
            Console.Write(result[i]);
    }
 
    // Driver code
    public static void Main()
    {
        reverses("internship at geeks for geeks");
    }
}
 
// This code is contributed by Sam007

Output:

skeegrofsk ee gtapi hsn retni

Time complexity -> O(N)
Auxiliary Space -> O(N)

Optimized Solution
The idea is to use two pointers to reverse.

C++

// C++ program to implement
// the above approach
#include <iostream>
using namespace std;
 
void preserveSpace(string &str)
{
    int n = str.length();
 
    // Initialize two pointers as two corners
    int start = 0;
    int end = n - 1;
 
    // Move both pointers toward each other
    while (start < end) {
 
        // If character at start or end is space,
        // ignore it
        if (str[start] == ' ') {
            start++;
            continue;
        }
        else if (str[end] == ' ') {
            end--;
            continue;
        }
 
        // If both are not spaces, do swap
        else {
            swap(str[start], str[end]);
            start++;
            end--;
        }
    }
}
 
// Driver code
int main()
{
    string str = "internship at geeks for geeks";
    preserveSpace(str);
    cout << str;
    return 0;
}

Java

// Java program to implement
// the above approach
import java.io.*;
import java.util.*;
 
class GFG{
     
public static void preserveSpace(String str)
{
    int n = str.length();
  
    // Initialize two pointers as two corners
    int start = 0;
    int end = n - 1;
     
    char[] Str = str.toCharArray();
     
    // Move both pointers toward each other
    while (start < end)
    {
         
        // If character at start or end 
        // is space, ignore it
        if (Str[start] == ' ') 
        {
            start++;
            continue;
        }
        else if (Str[end] == ' ') 
        {
            end--;
            continue;
        }
  
        // If both are not spaces, do swap
        else
        {
            char temp = Str[start];
            Str[start] = Str[end];
            Str[end] = temp;
            start++;
            end--;
        }
    }
    System.out.println(String.valueOf(Str));
}
 
// Driver Code
public static void main(String[] args) 
{
    String str = "internship at geeks for geeks";
     
    preserveSpace(str);
}
}
 
// This code is contributed by divyeshrabadiya07

Python3

# Python3 program to implement
# the above approach
def preserveSpace(Str):
   
    n = len(Str)
    Str = list(Str)
 
    # Initialize two pointers 
    # as two corners 
    start = 0
    end = n - 1
 
    # Move both pointers 
    # toward each other 
    while(start < end):
 
        # If character at start 
        # or end is space, 
        # ignore it 
        if(Str[start] == ' '):
            start += 1
            continue
        elif(Str[end] == ' '):
            end -= 1
            continue
             
        # If both are not 
        # spaces, do swap 
        else:
            Str[start], Str[end] = (Str[end],
                                    Str[start])
            start += 1
            end -= 1
    print(''.join(Str))
 
# Driver code 
Str = "internship at geeks for geeks"
preserveSpace(Str); 
 
# This code is contributed by avanitrachhadiya2155

C#

// C# program to implement
// the above approach
using System;
using System.Collections.Generic; 
 
class GFG{
     
static void preserveSpace(string str)
{
    int n = str.Length;
     
    // Initialize two pointers 
    // as two corners
    int start = 0;
    int end = n - 1;
      
    char[] Str = str.ToCharArray();
      
    // Move both pointers toward 
    // each other
    while (start < end)
    {
         
        // If character at start or 
        // end is space, ignore it
        if (Str[start] == ' ') 
        {
            start++;
            continue;
        }
        else if (Str[end] == ' ') 
        {
            end--;
            continue;
        }
   
        // If both are not spaces, do swap
        else
        {
            char temp = Str[start];
            Str[start] = Str[end];
            Str[end] = temp;
            start++;
            end--;
        }
    }
    Console.Write(new string(Str));
}
 
// Driver code    
static void Main()
{
    string str = "internship at geeks for geeks";
  
    preserveSpace(str);
}
}
 
// This code is contributed by divyesh072019

Output:

skeegrofsk ee gtapi hsn retni

This article is contributed by Rishabh Jain. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.

My Personal Notes

Related Articles

C++

Java

C#

C++

Java

Python3

C#