Reverse a string preserving space positions

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

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

Create a string to store result. Mark the space position of the given string in this string.
Insert the character from 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 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 use two pointers to reverse.

#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; 
} 

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.

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My Personal Notes

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

C++

Java

C#

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