二叉树的层级遍历

public class TreeNode {    int val = 0;    TreeNode left = null;    TreeNode right = null;    public TreeNode(int val) {        this.val = val;    }}public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {        ArrayList<Integer> result = new ArrayList<Integer>();        if(root == null)return result;        Queue<TreeNode> que = new LinkedList<TreeNode>();        //result.add(root.val);        que.add(root);        while(!que.isEmpty()){            TreeNode t = que.remove();            if(t.left != null)que.add(t.left);            if(t.right != null)que.add(t.right);            result.add(t.val);        }        return result;    }