Bubble Sort - C++

#include <iostream>
#include <string>
#include <vector>
#include <cstdlib>

using namespace std;

void bubblesort(vector<char>& vec) {
    int n = vec.size();

    // nest loops because O(n^2)
    // changes_made is an optimization that can reduce runtime in certain conditions
    bool changes_made = false;

    for (int i = 0; i < n - 1; i++) {

        for (int j = 0; j < n - 1; j++) {
            if (vec[j] > vec[j+1]) {

                // swap them
                char temp = vec[j];
                vec[j] = vec[j+1];
                vec[j+1] = temp;

                // tell program to run thru vec at least once more
                changes_made = true;
            }
        }

        // optimization to exit loop if no changes made
        if (!changes_made) break;

        changes_made = false;
    }
}

int main() {
    cout << "give me a string" << endl;
    string s; getline(cin, s);

    vector<char> vec(s.begin(), s.end());

    if (!vec.empty()) bubblesort(vec);

    string str(vec.begin(), vec.end());

    cout << str << endl;
    return 0;
}