How to find union of n vector?

Question

I have a 2-D vector of hyperedges as well as an adjacency list. I have to find the union of hyperEdges[i].size() vectors, but I can only find the union of just two vectors. What improvement can I make to my code below to do this? I want to store the union into newly declared 2-D vector connectedEdges

void find_union()
{
    connectedEdges.resize(nEdges+1);
    for(int i = 1; i <= nEdges; i++)
    {
        vector<int>::iterator it;
        connectedEdges[i].resize(nEdges+1);

        for(int j = 1; j < hyperEdges[i].size()-1; j++)
        {
            int p = hyperEdges[i][j-1];
            int q= hyperEdges[i][j];
            it = set_union(adjL[p].begin(), adjL[p].end(),adjL[q].begin(),adjL[q].end(), connectedEdges[i].begin());
        connectedEdges[i].resize(it-connectedEdges[i].begin());
        }
    }    
}

Example : {1,2,4,6,8}

{1,2,3,5,6}

{1,4,7,13,15}

Union of these three sets should be {1,2,3,4,5,6,7,8,13,15} But my program returns {1,2,3,4,5,6,8}

Answer 1

If you have a lot of vectors, I'd suggest to insert content of all of them into single std::set and then dump it back into std::vector.

Something like that:

std::vector<std::vector<int>> src = ...;
std::set<int> all;

for(int i = 0; i < src.size(); i++) {
    all.insert(src[i].begin(), src[i].end());
}

std::vector<int> result(all.begin(), all.end());