剑指offer--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<Integer> arrayList = new ArrayList<>();
        ArrayList<TreeNode> queue = new ArrayList<>();
        if ( root == null){
            return arrayList;
        }

        queue.add(root);
        for ( int i = 0 ; i < queue.size() ; i++){
            TreeNode node = queue.get(i);
            if ( node.left != null){
                queue.add(node.left);
            }
            if ( node.right != null){
                queue.add(node.right);
            }
            arrayList.add(node.val);
        }

        return arrayList;
    }
}