【剑指offer】从上往下打印二叉树

//import java.util.LinkedList;
//import java.util.Queue;

public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<TreeNode>();
ArrayList<Integer> listVal = new ArrayList<Integer>();
if (root == null)
return listVal;
queue.add(root);
listVal.add(root.val);
TreeNode node = root;
while (!queue.isEmpty()) {
node = queue.poll();
if (node.left != null) {
queue.add(node.left);
listVal.add(node.left.val);
}
if (node.right != null) {
queue.add(node.right);
listVal.add(node.right.val);
}
}
return listVal;
}