List of element adjancy

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C++

I have a grid in which every cell build up of 3 nodes. I need to find the neighboring cell of every single cell in the grid. The point is if two cells share the same edge they are adjacent. Based on this strategy I wrote the following code and it's working fine but as you can see it's not an efficient code as in one point i have 4 loops with one if condition. I know this can be done with using something like mapping and set but I can't figure out how. I would be grateful if you could help me with this.

What I have tried:

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

typedef std::vector<int>         VecInt_t;
typedef std::vector<VecInt_t>    VecVecInt_t;

void elm_adj(VecVecInt_t& connect);
int concat(int a, int b);

int main()
{
	VecVecInt_t Connectivity{ {0,1,3},{1,4,3},{1,5,4},{1,2,5},{3,7,6},{3,4,7},{4,5,7},{5,8,7} };
	elm_adj(Connectivity);
}

int concat(int a, int b)
{
	std::string s1 = std::to_string(a);
	std::string s2 = std::to_string(b);

	std::string s = s1 + s2;
	int c = stoi(s);

	return c;
}

void VecVec_no_duplicates_unique(VecVecInt_t& Vec)
{
	for (size_t i = 0; i < Vec.size(); i++) {
		std::sort(Vec[i].begin(), Vec[i].end());
		Vec[i].erase(std::unique(Vec[i].begin(), Vec[i].end()), Vec[i].end());
	}
}

void elm_adj(VecVecInt_t& connect) {

	VecVecInt_t arr(connect.size());

	for (int i = 0; i < connect.size(); i++) {

		int a = concat(connect[i][0], connect[i][1]);
		int b = concat(connect[i][1], connect[i][0]);
		int c = concat(connect[i][1], connect[i][2]);
		int d = concat(connect[i][2], connect[i][1]);
		int e = concat(connect[i][0], connect[i][2]);
		int f = concat(connect[i][2], connect[i][0]);
		arr[i].push_back(a);
		arr[i].push_back(b);
		arr[i].push_back(c);
		arr[i].push_back(d);
		arr[i].push_back(e);
		arr[i].push_back(f);
	}

	//for (int i = 0; i < arr.size(); i++) {
	//	for (int j = 0; j < arr[i].size(); j++) { // print all string in first vector of 'arr'
	//		std::cout << arr[i][j] << " ";
	//	}
	//	std::cout << std::endl;
	//}

	VecVecInt_t Elm_Adj(connect.size());

	int row = 0;
	int col = 0;

	for (int i = 0; i < arr.size(); i++) {
		for (row = 1; row < connect.size(); row++) {
			for (int j = 0; j < 6; j++) {

				for (col = 0; col < 6; col++) {

					if (arr[i][j] == arr[row][col]) {
						Elm_Adj[i].push_back(i);
						Elm_Adj[i].push_back(row);
					}
				}

			}
		}
	}

	VecVec_no_duplicates_unique(Elm_Adj);

	for (int i = 0; i < Elm_Adj.size(); i++) {
		for (int j = 0; j < Elm_Adj[i].size(); j++) {

			std::cout << Elm_Adj[i][j] << " ";
		}
		std::cout << std::endl;
	}
}