Determine if there are duplicates in vector

Question

I would like to determine if there are duplicates in a vector. What is the best option here?

sort(arrLinkToClients.begin(), arrLinkToClients.end(), [&](const type1& lhs,const type1& rhs)
{
    lhs.nTypeOfLink < rhs.nTypeOfLink;
});

auto it = unique(arrLinkToClients.begin(), arrLinkToClients.end(), [&](const type1& lhs, const type1& rhs)
{
    lhs.nTypeOfLink == rhs.nTypeOfLink;
});

//how to check the iterator if there are duplicates ?
if (it)
{
  //
}

Answer 1

The "best" option does not exist. It depends on how you define "best".

Here are some solutions, each with their own advantages and disadvantages:

Using map

template <class T>
auto has_duplicates(const std::vector<T>& v) -> bool
{
    std::unordered_map<T, int> m;

    for (const auto& e : v)
    {
        ++m[e];

        if (m[e] > 1)
            return true;
    }
    return false;
}

Using set

template <class T>
auto has_duplicates(const std::vector<T>& v) -> bool
{
    std::unordered_set<int> s;
    std::copy(v.begin(), v.end(), std::inserter(s, s.begin());
   
    return v.size() != s.size();
}

Using sort and adjacent_find (mutating range)

template <class T>
auto has_duplicates(std::vector<T>& v) -> bool
{
    std::sort(v.begin(), v.end());

    return std::adjacent_find(v.begin(), v.end()) != v.last();
}
   

Manual iteration with std::find

template <class T>
auto has_duplicates(const std::vector<T>& v) -> bool
{
    for (auto it = v.begin(); it != v.end(); ++it)
        if (std::find(it + 1, v.end(), *it) != v.end())
            return true;
            
    return false;
}

Manual iteration

template <class T>
auto has_duplicates(const std::vector<T>& v) -> bool
{
    for (auto i1 = v.begin(); i1 != v.end(); ++i1)
        for (auto i2 = i1 + 1; i2 != v.end(); ++i2)
            if (*i1 == *i2)
                return true;
    
    return false;
}