Memory Limit Exceeded in C++ but not in Python

Question

I am working on question 1249. Minimum Remove to Make Valid Parentheses on LeetCode. It is an easy question which I can solve with stack in both C++ and Python. However, my C++ version code leads to Memory Limit Exceeded, while my exact same Python code only consumes 15.3 MB memory. Could anyone help me understand what is wrong, please?

C++ code:

class Solution {
public:
    string minRemoveToMakeValid(string s) {
        stack<pair<char, int>> stack;
        for (int i=0; i<s.size();i++){
            if (s[i]!='(' && s[i]!=')')
                continue;
            else{
                if (!(stack.empty()) && stack.top().first=='(' && s[i]==')')
                    stack.pop();
                else
                    stack.push({s[i],i});
            } 
        }
        string res;
        for (int i=s.size()-1; i>=0;i--){ 
            if (!stack.empty() && i==stack.top().second)
                stack.pop();
            else{
                res = s[i] + res;
            }
        }
        return res;
            
    }
};

Even if replace stack data structure in C++ with vector, it still leads to Memory Limit Exceeded.

Python3

class Solution:
    def minRemoveToMakeValid(self, s: str) -> str:
        stack = []
        for i, c in enumerate(s):
            if c!="(" and c!=")":
                continue
            if stack and stack[-1][0] == '(' and c==")":
                stack.pop()
            else:
                stack.append([c,i])
        
        res =""
        for i in range(len(s)-1,-1,-1):
            if stack and i == stack[-1][1]:
                stack.pop()
            else:
                res = s[i] + res
        return res

Answer 1

This is another guess about what could reduce the memory footprint.

I guess that each res = s[i] + res; may repeatedly reallocate memory resulting in res being over allocated above what is needed.

The code below (I'm hoping) makes far fewer allocations, each of an exact size needed at the time:

class Solution {
public:
    std::string minRemoveToMakeValid(std::string s) {
        std::stack<std::pair<char, int>> stack;
        for (int i = 0; i < s.size(); i++) {
            if (s[i] != '(' && s[i] != ')')
                continue;
            else {
                if (!(stack.empty()) && stack.top().first == '(' && s[i] == ')')
                    stack.pop();
                else
                    stack.push({ s[i],i });
            }
        }
        std::string res(s.size(),' ');
        auto iter{ s.size() };
        for (int i = s.size() - 1; i >= 0; i--) {
            if (!stack.empty() && i == stack.top().second)
                stack.pop();
            else {
                //res = s[i] + res;
                res[--iter] = s[i];
            }
        }
        return res.substr(iter);

    }
};