Merkkijono

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

template <typename InputIt>
std::string remove(InputIt str_begin, InputIt begin, InputIt end) {
    std::string str;
    auto prefix_len = std::distance(str_begin, begin);
    if (prefix_len > 0) {
        str.reserve(std::distance(str_begin, begin));
        str.assign(str_begin, begin);
    }

    auto it = begin;
    while (begin != end && *begin == *it) {
        ++begin;
    }
    if (std::distance(str_begin, end) == 0) {
        return str;
    }
    str.append(begin, end);

    return str;
}

int main() {
    std::string str;
    std::cin >> str;

    for (auto it = str.begin() + 1; it != str.end() && !str.empty(); ++it) {
        /* std::cout << str << ", it: " << *it << ", prev: " << *(it - 1) */
        /*           << ", dist: " << std::distance(str.begin(), it) << '\n'; */
        if (*it == *(it - 1)) {
            str = remove(str.begin(), it - 1, str.end());
            it = str.begin();
        }
    }
    std::cout << str << '\n';
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED