从上往下打印二叉树

import java.util.ArrayList;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<TreeNode> queue = new ArrayList();
        ArrayList<Integer> arrayList = new ArrayList();
        queue.add(root);
        while(!queue.isEmpty()) {
            TreeNode node = queue.remove(0);
            if (null != node) {
                arrayList.add(node.val);
                if (null != node.left){
                    queue.add(node.left);
                }
                if (null != node.right){
                    queue.add(node.right);
                }
            }
        }
        return arrayList;
    }
}