Reverse a string preserving space positions

12 Jul, 2022
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.
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++
Java
Python3
C#
Javascript
// C++ program to reverse a string preserving// spaces.#include <iostream>using namespace std; // Function to reverse the string// and preserve the space positionstring 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.            while(result[j] == ' ')                j--;             result[j] = str[i];            j--;        }    }     return result;} // Driver codeint main(){    string str = "internship at geeks for geeks";    cout << reverses(str) << endl;    return 0;}
// 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
# Python3 program to reverse a string preserving# spaces. # Function to reverse the string# and preserve the space positiondef reverses(st):     # Mark spaces in result    n = len(st)    result = [0] * n         for i in range(n):        if (st[i] == ' '):            result[i] = ' '     # Traverse input string from beginning    # and put characters in result from end    j = n - 1    for i in range(len(st)):                 # Ignore spaces in input string        if (st[i] != ' '):                         # Ignore spaces in result.            if (result[j] == ' '):                j -= 1             result[j] = st[i]            j -= 1                 return ''.join(result) # Driver codeif __name__ == "__main__":     st = "internship at geeks for geeks"    print(reverses(st)) # This code is contributed by ukasp
// 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
<script>// Javascript program to reverse a string// preserving spaces. // Function to reverse the string    // and preserve the space positionfunction reverses(str){    let inputArray = str.split("");        let result = new Array(inputArray.length);          // Mark spaces in result        for (let i = 0; i < inputArray.length; i++) {            if (inputArray[i] == ' ') {                result[i] = ' ';            }        }          // Traverse input string from beginning        // and put characters in result from end        let j = result.length - 1;        for (let 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--;            }        }        document.write((result).join(""));}// driver functionreverses("internship at geeks for geeks");  // This code is contributed by ab2127</script>
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++
Java
Python3
C#
Javascript
// 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 codeint main(){    string str = "internship at geeks for geeks";    preserveSpace(str);    cout << str;    return 0;}
// Java program to implement// the above approachimport 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 Codepublic static void main(String[] args){    String str = "internship at geeks for geeks";         preserveSpace(str);}} // This code is contributed by divyeshrabadiya07
# Python3 program to implement# the above approachdef 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 codeStr = "internship at geeks for geeks"preserveSpace(Str); # This code is contributed by avanitrachhadiya2155
// C# program to implement// the above approachusing 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
<script>     // Javascript program to implement    // the above approach         function preserveSpace(str)    {        let n = str.length;         // Initialize two pointers        // as two corners        let start = 0;        let end = n - 1;         let Str = str.split('');         // 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            {                let temp = Str[start];                Str[start] = Str[end];                Str[end] = temp;                start++;                end--;            }        }        document.write(Str.join(""));    }         let str = "internship at geeks for geeks";       preserveSpace(str); </script>
skeegrofsk ee gtapi hsn retni
 
Time complexity: O(n) Auxiliary Space: O(1)
This article is contributed by Rishabh Jain. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. 
Sam007
Imam
avanitrachhadiya2155
divyeshrabadiya07
divyesh072019
suresh07
ukasp
ab2127
geekhelena
geekygirl2001
two-pointer-algorithm
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