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Reverse a string without affecting special characters

  • Difficulty Level : Easy
  • Last Updated : 26 Sep, 2021

Given a string, that contains a special character together with alphabets (‘a’ to ‘z’ and ‘A’ to ‘Z’), reverse the string in a way that special characters are not affected.
Examples: 
 

Input:   str = "a,b$c"
Output:  str = "c,b$a"
Note that $ and , are not moved anywhere.  
Only subsequence "abc" is reversed

Input:   str = "Ab,c,de!$"
Output:  str = "ed,c,bA!$"

 

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Simple Solution: 
1) Create a temporary character array say temp[]. 
2) Copy alphabetic characters from the given array to temp[]. 
3) Reverse temp[] using standard string reversal algorithm
4) Now traverse input string and temp in a single loop. Wherever there is an alphabetic character is input string, replace it with the current character of temp[].
Efficient Solution: 
Time complexity of the above solution is O(n), but it requires extra space, and it does two traversals of an input string. 
We can reverse with one traversal and without extra space. Below is the algorithm. 
 



1) Let input string be 'str[]' and length of string be 'n'
2) l = 0, r = n-1
3) While l is smaller than r, do following
    a) If str[l] is not an alphabetic character, do l++
    b) Else If str[r] is not an alphabetic character, do r--
    c) Else swap str[l] and str[r]

Below are implementations of the above algorithm.
 

C++




// C++ program to reverse a string
// with special characters
#include<bits/stdc++.h>
using namespace std;
 
// Returns true if x is an alphabetic
// character, false otherwise
bool isAlphabet(char x)
{
    return ( (x >= 'A' && x <= 'Z') ||
            (x >= 'a' && x <= 'z') );
}
 
void reverse(char str[])
{
    // Initialize left and right pointers
    int r = strlen(str) - 1, l = 0;
 
    // Traverse string from both ends until
    // 'l' and 'r'
    while (l < r)
    {
        // Ignore special characters
        if (!isAlphabet(str[l]))
            l++;
        else if(!isAlphabet(str[r]))
            r--;
 
        else // Both str[l] and str[r] are not special
        {
            swap(str[l], str[r]);
            l++;
            r--;
        }
    }
}
 
// Driver code
int main()
{
    char str[] = "a!!!b.c.d,e'f,ghi";
    cout << "Input string: " << str << endl;
    reverse(str);
    cout << "Output string: " << str << endl;
    return 0;
}

Java




// Java code to illustrate how to reverse
// an array without affecting special characters.
class GFG
{
    public static void reverse(char str[])
    {
        // Initialize left and right pointers
        int r = str.length - 1, l = 0;
 
        // Traverse string from both ends until
        // 'l' and 'r'
        while (l < r)
        {
            // Ignore special characters
            if (!Character.isAlphabetic(str[l]))
                l++;
            else if(!Character.isAlphabetic(str[r]))
                r--;
 
            // Both str[l] and str[r] are not spacial
            else
            {
                char tmp = str[l];
                str[l] = str[r];
                str[r] = tmp;
                l++;
                r--;
            }
        }
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        String str = "a!!!b.c.d,e'f,ghi";
        char[] charArray = str.toCharArray();
 
        System.out.println("Input string: " + str);
                            reverse(charArray);
        String revStr = new String(charArray);
 
        System.out.println("Output string: " + revStr);
    }
}
 
// This code is contributed by panwarabhishek345

Python3




def reverseSting(text):
    index = -1
 
    # Loop from last index until half of the index    
    for i in range(len(text)-1, int(len(text)/2), -1):
 
        # match character is alphabet or not        
        if text[i].isalpha():
            temp = text[i]
            while True:
                index += 1
                if text[index].isalpha():
                    text[i] = text[index]
                    text[index] = temp
                    break
    return text
     
# Driver code
string = "a!!!b.c.d,e'f,ghi"
print ("Input string: ", string)
string = reverseSting(list(string))
print ("Output string: ", "".join(string))
  
# This code is contributed by shiva9610

C#




// C# code to illustrate how to reverse
// an array without affecting special characters.
using System;
public class GFG
{
    public static void reverse(char []str)
    {
        // Initialize left and right pointers
        int r = str.Length - 1, l = 0;
 
        // Traverse string from both ends until
        // 'l' and 'r'
        while (l < r)
        {
            // Ignore special characters
            if (!char.IsLetter(str[l]))
                l++;
            else if(!char.IsLetter(str[r]))
                r--;
 
            // Both str[l] and str[r] are not spacial
            else
            {
                char tmp = str[l];
                str[l] = str[r];
                str[r] = tmp;
                l++;
                r--;
            }
        }
    }
 
    // Driver Code
    public static void Main()
    {
        String str = "a!!!b.c.d,e'f,ghi";
        char[] charArray = str.ToCharArray();
 
        Console.WriteLine("Input string: " + str);
                            reverse(charArray);
        String revStr = new String(charArray);
 
        Console.WriteLine("Output string: " + revStr);
    }
}
 
// This code is contributed by PrinciRaj1992

Javascript




<script>
 
// JavaScript program to reverse a string
// with special characters
 
// Returns true if x is an alphabetic
// character, false otherwise
function isAlphabet( x)
{
    return ( (x >= 'A' && x <= 'Z') ||
             (x >= 'a' && x <= 'z') );
}
function swap(str,a,b)
{
    var c="";
    for(var i=0;i<str.length;i++)
    {
        if(i==a)
         c= c+ str[b];
         else
            if(i==b)
                c=c+str[a];
               else
               c=c+str[i];
             
    }
    return c;
   }
     
 
function reverse( str)
{
    // Initialize left and right pointers
    var r = str.length - 1, l = 0;
 
    // Traverse string from both ends until
    // 'l' and 'r'
    while (l < r)
    {
        // Ignore special characters
        if (!isAlphabet(str[l]))
            l++;
        else if(!isAlphabet(str[r]))
            r--;
 
        else // Both str[l] and str[r] are not spacial
        {
            str=swap(str,l, r);
            l++;
            r--;
        }
    }
    document.write("Output string: "+ str +"<br>");
}
 
 
    var str= "a!!!b.c.d,e'f,ghi";
    document.write( "Input string: " + str +"<br>");
    reverse(str);
    
  
</script>

Output:  

Input string: a!!!b.c.d,e'f,ghi
Output string: i!!!h.g.f,e'd,cba

 

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