排序向量

Question

I have a vector of vectors of int like this:

std::vector<std::vector<int>> vec_vec{{3,5,1,6},{2,4},...};

The results should be

 Case1:   {{1, 2, 3, 4}, {5, 6}} 
 Case2:  {1,2,3,4,5,6}
 Case3:  {{1, 3, 5, 6}, {2, 4}} 

I found many ways to do this, the best one I found need complexity O(n^2) to sort them.

What's the best complexity for the case1 , case2 and case3 ?

So what's the best way to write a native (c++11,c++14) cross platform code to sort that's vector? is O(n^2) is the best complexity? The memory is important also.

I checked this solution here , but it seems it also took O(n^2) to sort the vectors?

Answer 1

The easiest case is case 2. To solve that, make a temporary new vector, move all elements there, and sort the new vector.

std::vector<int> temp;
for (const auto& vec: vec_vec)
{
    std::copy(vec.begin(), vec.end(), std::back_inserter(temp));
}
std::sort(temp.begin(), temp.end());

The copying part is O(n); the sorting part is O(n log(n)).

To implement case 1, copy the elements from temp back to vec_vec - this should be easy.