Efficient method to get node connectivity in FEM - c

Question

I have information of element connectivity in problem of finite element method.
For example,
1, 5, 6, 8, 7, 1, 2, 4, 3
The first column is the index of element, and the last 8 columns are the node index which composed entries for element 1.
The goal what I want is node array like as below.
It is an array with all the node indices associated with each node.
node_arr[1] = [2 3 4 5 6 7 8]
node_arr[2] = [1 3 4 5 6 7 8]
node_arr[3] = [1 2 4 5 6 7 8]
node_arr[4] = [1 2 3 5 6 7 8]
node_arr[5] = [1 2 3 4 6 7 8]
node_arr[6] = [1 2 3 4 5 7 8]
node_arr[7] = [1 2 3 4 5 6 8]
node_arr[8] = [1 2 3 4 5 6 7]
So the current state of my code is as below .
1-count the number about how many elements contain that node index
for loop i (entire node){
for loop j (entire element){
if (element j has node i is true){
node_count[i] = node_count[i] + 1;
}
}
}
2-Allocate node_node_arr to contain target information by using the results of step 1
node_node_arr = malloc(by using node_count)
3-Again, it searches the entire element node index and append the node index related with current node index for composing node_node_array.
for loop i (entire node){
for loop j (entire element){
if (element j has node i is true){
node_node_arr[i] append node indices;
}
}
}
However this approach is so slow .
If both the number of elements and the number of nodes increase, the calculation time increases tremendously.
I need speed up.
Please help me.