How to make elements of vector unique? (remove non adjacent duplicates)

Question

I have a vector containing few non-adjacent duplicates.

As a simple example, consider:

2 1 6 1 4 6 2 1 1

I am trying to make this

Answer 1

As the question was "is there any STL algorithm...? what is its complexity?" it makes sense to implement the function like std::unique:

template 
inline FwdIterator stable_unique(FwdIterator first, FwdIterator last)
{
    FwdIterator result = first;
    std::unordered_set seen;

    for (; first != last; ++first)
        if (seen.insert(*first).second)
            *result++ = *first;
    return result;
}

So this is how std::unique is implemented plus an extra set. The unordered_set shall be faster than a regular set. All elements are removed that compare equal to the element right preceding them (the first element is kept because we cannot unify to nothing). The iterator returned points to the new end within the range [first,last).

EDIT: The last sentence means that the container itself is NOT modified by unique. This can be confusing. The following example actually reduces the container to the unified set.

1: std::vector v(3, 5);
2: v.resize(std::distance(v.begin(), unique(v.begin(), v.end())));
3: assert(v.size() == 1);

Line 1 creates a vector { 5, 5, 5 }. In line 2 calling unique returns an iterator to the 2nd element, which is the first element that is not unique. Hence distance returns 1 and resize prunes the vector.