4.4 Given a binary tree, design an algorithm which creates a linked list of all the nodes at each depth (e.g., if you have a tree with depth D,you'll have D linked lists).

4.4 treeDepth2LinkedList

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    
    TreeNode(int val) {
        this.val = val;
        left = null;
        right = null;
    }
}

public ArrayList<LinkedList<TreeNode>> treeDepth2LinkedList(TreeNode root) {
    if (root == null) {
        return null;
    }
    
    ArrayList<LinkedList<TreeNode>> result = new ArrayList<LinkedList<TreeNode>>();
    Queue<TreeNode> queue = new LinkedList<TreeNode>();
    queue.add(root);
    
    while (!queue.isEmpty()) {
        int count = queue.size();
        LinkedList<TreeNode> depth = new LinkedList<TreeNode>();
        
        while (count > 0) {
            TreeNode cur = queue.remove();
            
            depth.add(cur);
            if (cur.left != null) {
                queue.add(cur.left);
            }
            if (cur.right != null) {
                queue.add(cur.right);
            }
            
            count--;
        }
        
        result.add(depth);
    }
    
    return result;
}