Reverse an array without affecting special characters

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


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 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 alphabetic character is input string, replace it with current character of temp[].

Efficient Solution:
Time complexity of above solution is O(n), but it requires extra space and it does two traversals of input string.
We can reverse with one traversal and without extra space. Below is 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 is C++ implementation of above algorithm.


// C++ program to reverse a string with special characters
using namespace std;

// Returns true if x is an aplhabatic 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]))
        else if(!isAlphabet(str[r]))

        else // Both str[l] and str[r] are not spacial
            swap(str[l], str[r]);

// Driver program
int main()
    char str[] = "a!!!b.c.d,e'f,ghi";
    cout << "Input string: " << str << endl;
    cout << "Output string: " << str << endl;
    return 0;


# Python program to reverse a string with special characters

# Returns true if x is an aplhabatic character, false otherwise
def isAlphabet(x):
	return x.isalpha()

def reverse(string):
	LIST = toList(string)

	# Initialize left and right pointers
	r = len(LIST) - 1
	l = 0

	# Traverse LIST from both ends until
	# 'l' and 'r'
	while l < r:

		# Ignore special characters
		if not isAlphabet(LIST[l]):
			l += 1
		elif not isAlphabet(LIST[r]):
			r -= 1

		else:	# Both LIST[l] and LIST[r] are not special
			LIST[l], LIST[r] = swap(LIST[l], LIST[r])
			l += 1
			r -= 1

	return toString(LIST)

# Utility functions
def toList(string):
	List = []
	for i in string:
	return List

def toString(List):
	return ''.join(List)

def swap(a, b):
	return b, a

# Driver program
string = "a!!!b.c.d,e'f,ghi"
print "Input string: " + string
string = reverse(string)
print "Output string: " + string
# This code is contributed by Bhavya Jain


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

Thanks to anonymous user for suggesting above solution here.

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