简单呢

#include <iostream>
#include <unordered_set>
using namespace std;
int main()
{
    string str;
    cin >> str;
    unordered_set<char> s;
    for (int i = str.size()-1; i != -1; i--) {
        if (s.find(str[i]) == s.end()) {
            cout << str[i];
            s.emplace(str[i]);
        }
    }
}