pat-1084.cpp

#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <unordered_map>
#include <limits>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>

int main() {
    std::string org;
    std::string type;
    std::cin >> org >> type;

    auto trans_pred = [](char ch) { return std::toupper(ch); };
    std::transform(std::begin(org), std::end(org), std::begin(org), trans_pred);
    std::transform(std::begin(type), std::end(type), std::begin(type), trans_pred);

    std::set<char> memed;
    auto mem_print = [&](char ch) {
        if (!memed.count(ch)) {
            std::cout << ch;
            memed.insert(ch);
        }
    };
    size_t j = 0;
    for (size_t i = 0; i < type.size(); ++i, ++j) {
        if (type[i] != org[j]) {
            while (type[i] != org[j]) {
                mem_print(org[j++]);
            }
        }
    }
    while (j < org.size()) {
        mem_print(org[j++]);
    }

    return 0;
}

#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <unordered_map>
#include <limits>
#include <map>
#include <iterator>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>

int main() {
    std::string org;
    std::string type;
    std::cin >> org >> type;

    auto trans_pred = [](char ch) { return std::toupper(ch); };
    std::transform(std::begin(org), std::end(org), std::begin(org), trans_pred);
    std::transform(std::begin(type), std::end(type), std::begin(type), trans_pred);

    std::map<char, size_t> mem_meet;
    for (size_t i = 0; i < org.size(); ++i) {
        char ch = org[i];
        if (!mem_meet.count(ch)) {
            mem_meet[ch] = i;
        }
    }
    std::set<char> set_org(std::begin(org), std::end(org));
    std::set<char> set_type(std::begin(type), std::end(type));
    std::vector<char> diff_res;
    std::set_difference(std::begin(set_org), std::end(set_org),
            std::begin(set_type), std::end(set_type),
            std::back_inserter(diff_res));
    std::sort(std::begin(diff_res), std::end(diff_res),
            [&](const char& lhs, const char& rhs) {
                return mem_meet[lhs] < mem_meet[rhs];
            });
    std::copy(std::begin(diff_res), std::end(diff_res),
            std::ostream_iterator<char>(std::cout, ""));

    return 0;
}