3 Graph representations - Object/Pointer, Adjacency List, Adjacency Matrix

adjList.java

// Uses Node class like below
/*Node(int val) {
  Node next;
  int val = val;
}*/
public HashMap<Integer,Node<Integer>> adjList(Pair pairArr) {
  if (pairArr.length == 0) {
    return null;
  }

  HashMap<Integer,Node<Integer>> nodeMap = new HashMap<Integer,Node<Integer>>();
  for (Pair p : pairArr) {
      if (!nodeMap.containsKey(p.firstInt)) {
        Node firstNode = new Node(p.secondInt);
        nodeMap.put(p.firstInt, firstNode); 
      } else {
        Node currNode = nodeMap.get(p.firstInt);
        Node newNode = new Node(p.secondInt);
        newNode.next = currNode;
        nodeMap.put(p.firstInt,newNode);
      }
  }
  return nodeMap;
}

adjMatrix.java

public int[][] adjMatrix(Pair pairArr, boolean directed) {
	if (pairArr.length == 0) {
		return null;
	}
	
	int connected = 1;
	int[][] adjMatrix = new int[pairArr.length][pairArr.length];
	// has to make sure if directed or undirected just in case it ignores edges in undirected case
	if (directed == true) {
		for (Pair p : pairArr) {
			int[p.firstInt][p.secondInt] = connected;
		}
	} else { // in the case its undirected and doesn’t write in pairArr both connected cases
		for (Pair p: pairArr) {
			int[p.firstInt][p.secondInt] = connected;
			int[p.secondInt][p.firstInt] = connected;
		}
	}
	return adjMatrix;
}

objPointer.java

// Uses Node class like below
Node(int val) {
  List<Node> neighbours;
  Int val = val;
}


// Didn’t want to do this similarly to my adjacency list hashtable implementation so 
// tried to go through pairArr and add newNode whenever hit a similar firstInt	
public ArrayList<Node> objPointer(Pair[] pairArr) {

	if (pairArr.length == 0) {
		return null;
	}
	
	ArrayList<Node> nodeList = new ArrayList<Node>();
	for (int i = 0; i < pairArr.length; i++) {
		int checkInt = pairArr[i].firstInt;
		Node newNode = new Node(checkInt) 
		for (Pair p: pairArr) {
			if (checkInt == p.firstInt) { 
        Node connectedNode = new Node(p.secondInt);
				newNode.neighbours.add(connectedNode);
			}
		}
		nodeList.add(newNode);
	}
	return nodeList;
}