Special array reversal

Given a string S, containing special characters and all the alphabets, reverse the string without

affecting the positions of the special characters.

• Time : O(n)
• Space : O(1)

class Solution {
    bool isAlpha(char ch) {
        return islower(ch) or isupper(ch);
    }
public:
    string reverse(string str) {
        int l = 0, r = str.length() - 1;
        while(l < r) {
            while(l < r and !isAlpha(str[l])) l++;
            while(l < r and !isAlpha(str[r])) r--;
            if(l < r)
                swap(str[l++], str[r--]);
        }
        return str;
    }
};