題目描述
從上往下打印出二叉樹的每個節點，同層節點從左至右列印。

import java.util.ArrayList;
import java.util.Queue;
import java.util.LinkedList;

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