#include <iostream>
#include <vector>
#include <algorithm>

int main()
{
    std::vector<int> test{5, 3, 8, 4, -1, 1, 11, 9, 6};
    // get the position of -1
    auto itr = std::find(test.begin(), test.end(), -1);
    // sort all elements so that -1 will be moved to end of vector
    std::sort(test.begin(), test.end(), [](const int& lhs, const int& rhs )
        {
            if( lhs == -1 ) return false;
            if( rhs == -1 ) return true;
            return lhs < rhs;
        });

    test.erase(test.end()-1);   //  now erase it from end
    test.insert(itr, -1);       //  add to the erlier position

    for(const auto& it: test)
        std::cout << it << " ";

    return 0;
}