BSTLevelOrderTraversal.java

public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if (root == null) return result;

        Queue<TreeNode> nodeQ = new LinkedList<TreeNode>();
        TreeNode dummy = new TreeNode(0);
        nodeQ.add(root);
        nodeQ.add(dummy);
        while(nodeQ.size() > 0){
            ArrayList<Integer> level = new ArrayList<Integer>();
            while(nodeQ.peek() != dummy){
                TreeNode temp = nodeQ.poll();
                level.add(temp.val);
                if (temp.left != null) nodeQ.add(temp.left);
                if (temp.right != null) nodeQ.add(temp.right);
            }
            result.add(level);
            if (nodeQ.size() == 1 && nodeQ.peek() == dummy) break;
            nodeQ.add(nodeQ.poll());
        }
        return result;
    }
}