把string当栈用，扫一遍就可以了，时间复杂度O(n)

#include <iostream>
#include <string>
using namespace std;

int main() {
    string s;
    cin >> s;
    int n = s.size();
    string st;
    for (int i = 0; i < n; ++i) {
        if (st.empty() || st.back() != s[i]) {
            st.push_back(s[i]);
        } else {
            st.pop_back();
        }
    }
    if (st.size() == 0) cout << 0;
    else cout << st;
    return 0;
}