Parenthesis checker, C++ (gcc)

Parenthesis checker

//Title of this code

#include <iostream>
#include <string>
#include <list>

using namespace std;

bool matchParenthesis(char a, char b) {
 
    if (a == '(' && b == ')')
        return true;  
    if (a == '[' && b == ']')
        return true;
    if (a == '{' && b == '}')
        return true;
    if (a == '<' && b == '>')
        return true;
    
    return false;
    
}

// () {} [] <>
bool checkMultiParenthesis(const string& s) {
    
    list<char> stack;
    
    for (int i = 0; i < s.length(); ++i) {
     
        // opener
        if (s[i] == '(' || s[i] == '{' || s[i] == '[' || s[i] == '<')
            stack.push_back(s[i]);
        
        
        // closer
        if (s[i] == ')' || s[i] == '}' || s[i] == ']' || s[i] == '>') {
            
            if (stack.empty() || !matchParenthesis(stack.back(), s[i]))
                return false;
               
            stack.pop_back();
                
        }
    }
    
    return true;
}

//eg. ((alf)ls) ? valid
//    )( (dkk)() ? invalid
//
//   )( )(( ))(
//

bool checkParentheses(const string& s) {

bool foundFirst = false;
        
    int sum = 0; 
    int i = 0;
    while (i < s.length()) {

if (!foundFirst && s[i] == ')')
            return false;
        
        else if (s[i] == ')')
            --sum;
        
        else if (s[i] == '(') {
            foundFirst = true;
            ++sum;
        }

++i;
    }
    
    return !sum;
}

int main()
{
    std::cout << boolalpha << checkParentheses("a(a)a(a)a") << endl;
    std::cout << boolalpha << checkParentheses("ae()w()(a()()r()(d)()()g()(e))()aa") << endl;
    
    
    cout << boolalpha << checkMultiParenthesis(" < ( [] { {}{<>} } ) >    ") << endl;
    
}


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