Valid Palindrome Solutions in C++

// Source : https://oj.leetcode.com/problems/valid-palindrome/
// Author : Hao Chen
// Date   : 2014-06-26



class Solution {
public:
    bool isPalindrome(string s) {
        s = removeNoise(s);
        for(int i=0; i<s.size()/2; i++){
            if (s[i]!= s[s.size()-i-1]){
                return false;
            }
        }
        return true;
    }
    
    string removeNoise(string& s){
        string d;
        for(int i=0; i<s.size(); i++){
            if(::isalpha(s[i]) || ::isdigit(s[i])){
                d.push_back(::tolower(s[i]));
            }
        }
        return d;
    }
};

#include <string>
using std::string;
#include <cctype>

class Solution {
public:
    bool isPalindrome(string s) {
        for (auto b=s.cbegin(), e=std::prev(s.cend()); b < e; ++b, --e) {
            while (!isalnum(*b)) ++b;
            while (!isalnum(*e)) --e;
            if (b < e && tolower(*b) != tolower(*e))
                return false;
        }
        return true;
    }
};

/// Source : https://leetcode.com/problems/valid-palindrome/description/
/// Author : liuyubobobo
/// Time   : 2018-05-27

#include <iostream>

using namespace std;


/// Two Pointers
/// Time Complexity: O(n)
/// Space Complexity: O(1)
class Solution {

public:
    bool isPalindrome(string s) {

        int i = next_alpha_numeric(s, 0);
        int j = prev_alpha_numeric(s, s.size() - 1);
        while(i <= j){
            if(tolower(s[i]) != tolower(s[j]))
                return false;
            i = next_alpha_numeric(s, i + 1);
            j = prev_alpha_numeric(s, j - 1);
        }
        return true;
    }

private:
    int next_alpha_numeric(const string& s, int index){
        for(int i = index ; i < s.size() ; i ++)
            if(isalnum(s[i]))
                return i;
        return s.size();
    }

    int prev_alpha_numeric(const string& s, int index){
        for(int i = index ; i >= 0 ; i --)
            if(isalnum(s[i]))
                return i;
        return -1;
    }
};


void print_bool(bool res){
    cout << (res ? "True" : "False") << endl;
}

int main() {

    string s1 = "A man, a plan, a canal: Panama";
    print_bool(Solution().isPalindrome(s1));

    string s2 = "race a car";
    print_bool(Solution().isPalindrome(s2));

    return 0;
}