Reverse a string without affecting special characters

• Difficulty Level : Easy
• Last Updated : 19 Apr, 2022

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!\$"```

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 ``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;``}` `// This code is contributed by Sania Kumari Gupta`

C

 `// C program to reverse a string with special characters``#include ``#include ``#include ``// 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--;``        ``// Both str[l] and str[r] are not special``        ``else` `{``            ``char` `temp = str[l];``            ``str[l] = str[r];``            ``str[r] = temp;``            ``// swap(str[l], str[r]);``            ``l++;``            ``r--;``        ``}``    ``}``}` `// Driver code``int` `main()``{``    ``char` `str[] = ``"a!!!b.c.d,e'f,ghi"``;``    ``printf``(``"Input string: %s\n"``, str);``    ``reverse(str);``    ``printf``(``"Output string: %s\n"``, str);``    ``return` `0;``}` `// This code is contributed by Sania Kumari Gupta`

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 Sania Kumari Gupta`

Python3

 `def` `reverseString(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

 ``

Output:

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