Couples and Single

A vector of integers is given. All values are in the range [1..100,000,000] and present in couples, but one that is single. Return it.

A C++11 GoogleTest test case to clarify the problem:

TEST(CouplesAndSingle, Simple)
{
    ASSERT_EQ(42, solution({ 1, 42, 1 }));
}

My solution uses an hash table where I store each new value value I see in input. If I see it a second time, I remove it from the table. In the end, I should have just one element in the hash table, the single value. It is not required, however I decided to return 0 in case of unexpected input.

Here is how I have implemented it:

int solution(const std::vector<int>& input)
{
    std::unordered_set<int> buffer; // 1
    for(int value : input) // 2
    {
        if(value < 1 || value > 100000000) // 3
            return 0;

        auto it = buffer.find(value); // 4
        if(it == buffer.end())
            buffer.insert(value);
        else
            buffer.erase(it);
    }
    return (buffer.size() != 1) ? 0 : *buffer.begin(); // 5
}

1. STL unordered_set implements hash table in C++11.
2. For each element in input.
3. Paranoid check.
4. If the current value is not in buffer, insert it. Otherwise remove it.
5. The single value is the first (and unique) element in buffer.