Find which numbers appears most in a vector

Question

I have some numbers stored in a vector . I want to find which number appears most in the vector.

Is there any easy/fast algorithm (STL or whatever) that does this ?<

Answer 1

Here is an O(n) generic solution for finding the most common element in an iterator range. You use it simply by doing:

int commonest = most_common(my_vector.begin(), my_vector.end());

The value type is extracted from the iterator using iterator_traits<>.

template::value_type>
T most_common(InputIt begin, InputIt end)
{
    std::map counts;
    for (InputIt it = begin; it != end; ++it) {
        if (counts.find(*it) != counts.end()) {
            ++counts[*it];
        }
        else {
            counts[*it] = 1;
        }
    }
    return std::max_element(counts.begin(), counts.end(),
            [] (const std::pair& pair1, const std::pair& pair2) {
            return pair1.second < pair2.second;})->first;
}