Why is this code not working if I add an alphabet in parantheses?

Question

#include <iostream>
#include <stack>

bool is_valid_parenthesis(std::string str)
{
std::stack<char> st;
for (char c: str) {
if (c == '(') {
st.push(')');
} else if (c == '{') {
st.push('}');
} else if (c == '[') {
st.push(']');
} else if (!st.empty()) {
if (st.top() != c) {
return false;
} else {
st.pop();
}
}
}
return st.empty();
}

int main()
{
std::string str1{"(a)[]{}"};
if (is_valid_parenthesis(str1)) {
std::cout << str1 << " has valid parenthesis." << std::endl;
} else {
std::cout << str1 << " does not have valid parenthesis." << std::endl;
}
std::string str2{"([)]"};
if (is_valid_parenthesis(str2)) {
std::cout << str2 << " has valid parenthesis." << std::endl;
} else {
std::cout << str2 << " does not have valid parenthesis." << std::endl;
}
return 0;
}

Answer 1

For the alphabets it doesn't have a pair so it's returning false.... Please keep a condition for c is a bracket and ignore all alphanumeric.

https://code.sololearn.com/cj6456rjwzx1/?ref=app