int lonelyinteger(vector<int> a) {

    // Use a set to track numbers we've encountered as we iterate through
    set<int> b;
    
    for (int i : a) {
        // If we lookup i and it doesn't return the end iterator
        if( b.find( i ) != b.end() )
        {
            b.erase( i ); // Remove the already encountered integer
        } else {
            b.emplace( i ); // Track encountered integer
        }
    }
    
    // Convert set to vector
    vector<int> b_vect( b.begin(), b.end() );
    
    // There should only be one element left...
    return b_vect.at(0);
}