L = new int[mid]; delete[] L;
for arrays (which is what you want) or
L = new int; delete L;
for single elements.
But it’s more simple to use vector, or use smartpointers, then you don’t have to worry about memory management.
std::vector<int> L(mid);
L.data() gives you access to the int[] array buffer and you can L.resize() the vector later.
auto L = std::make_unique<int[]>(mid);
L.get() gives you a pointer to the int[] array.