why SIGSEGV runtime error?

Could someone please tell me why this is giving me a runtime error (SIGSEGV). I tried to implement my own quicksort rather than the C++ standard sort for the problem, but it ends up giving the runtime error. When I use the C++ sort, it doesn't return an error, so the problem has to be in my version of the sort.

The link to my full solution is here: http://www.codechef.com/users/shanman17

The problem is here: http://www.codechef.com/problems/BUYING2

Here's my code for the sort:

vector<int> concat(vector<int> low, vector<int> high){
    low.insert(low.end(), high.begin(), high.end());
    return low;
}

vector<int> quicksort(vector<int> array){
    int size = array.size(), pivot = (array[0] + array[size - 1]) / 2;
    if (size > 1){
        vector<int> low;
        vector<int> high;
        for (int a = 0; a < size; a++){
            if (array[a] <= pivot){
                low.push_back(array[a]);
            }
            else if (array[a] > pivot){
                high.push_back(array[a]);
            }
        }
        return concat(quicksort(low), quicksort(high));
    }
    else {
        return array;
    }
}