从上向下打印二叉树

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
/**
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> list = new ArrayList<Integer>();
        
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        
        queue.offer(root); //根结点入queue
        
        while(root != null){
            TreeNode node = queue.poll(); //结点出queue
            list.add(node.val);
            
            //结点的左右孩子入queue
            if(root.left != null){
                queue.offer(root.left);
            }
            if(root.right != null){
                queue.offer(root.right);
            }
            
            root = queue.peek();
        }

        return list;
    
    }
}