java 代码

class Solution {
    public List<Integer> printFromTopToBottom(TreeNode root) {
        List<Integer> result = new ArrayList();
        if(root==null){
            return result;
        }
        LinkedList<TreeNode> list = new LinkedList();//先进先出
        list.add(root);
        while(list.size()>0){
            TreeNode tmp = list.get(0);
            result.add(tmp.val);
            if(tmp.left !=null) list.add(tmp.left);
            if(tmp.right !=null) list.add(tmp.right);
            list.pop();
        }
        return result;
    }


}