What am I doing wrong with the Java 8 PriorityQueue comparator?

Question

I'm trying to solve this exercise and here's my solution. It basically holds a tree map to map the nodes at the same veritical offset to a key. And uses a priority queue to split ties when there are multiple keys at the same (horizontal level) using the value at the node.

public List<List<Integer>> verticalTraversal(TreeNode root) {
    Map<Integer, PriorityQueue<Node>> map = new TreeMap<>();
    List<List<Integer>> out = new ArrayList<>();
    if(root == null)
        return out;
    Queue<Node> q = new LinkedList<>();
    Node r = new Node(root, 0, 0);
    q.add(r);
    while(!q.isEmpty()) {
        Node curr = q.remove();
        int x = curr.x;
        int y = curr.y;
        PriorityQueue<Node> pq = map.getOrDefault(y, new PriorityQueue<Node>((a,b) ->(a.x == b.x? a.t.val - b.t.val: a.x - b.x)));
        pq.add(curr);
        map.put(y,pq);
        if(curr.t.left!=null){
            Node left = new Node(curr.t.left, x+1, y-1);
            q.add(left);
        }
        if(curr.t.right!=null){
            Node right = new Node(curr.t.right, x+1, y + 1);
            q.add(right);
        }
    }
for (Map.Entry<Integer, PriorityQueue<Node>> entry : map.entrySet()){
   PriorityQueue<Node> pq = entry.getValue();
    List<Integer> vals = new ArrayList<>();
   for (Node pqNode: pq){
       vals.add(pqNode.t.val);                       

   }
out.add(new ArrayList<Integer>(vals));

}
return out;
}




class Node {
    TreeNode t;
    int y;
    int x;
    Node(TreeNode t, int x, int y) {
        this.t = t;
        this.x = x;
        this.y = y; 
    }
}

}

And to be clear here's where I think the problem is

  PriorityQueue<Node> pq = map.getOrDefault(y, new PriorityQueue<Node>((a,b) ->(a.x == b.x? a.t.val - b.t.val: a.x - b.x)));

I get the expected order when a.x isnt equal to b.x but it doesnt seem to go by the val when they're equal.

Here's the failing test case Actual : [[7,9],[5,6],[0,2,4],[1,3],[8]] Expected: [[9,7],[5,6],[0,2,4],[1,3],[8]]

Answer 1

What your doing wrong is that you iterate over the elements of the priority queue instead of polling it.

The documentation of PriorityQueue#iterator() clearly states:

Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order.

Instead of writing

for (Node pqNode: pq){
    vals.add(pqNode.t.val);                       
}

you should use:

Node pqNode;
while ((pqNode = pq.poll()) != null) {
    vals.add(pqNode.t.val);                       
}