剑指offer二十二之从上往下打印二叉树

import java.util.ArrayList;
import java.util.LinkedList;
/**
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<Integer>();
        
       
        //新建一个队列，LinkedList实现了Quene接口，可以直接当作队列来用
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();

        TreeNode current;  //当前节点
        if(root!=null){
        queue.offer(root);//根节点入队列
        }
        while (!queue.isEmpty()) {
            current = queue.poll();  //取出队列的头节点
            arrayList.add(current.val);//保存队列的头节点的值
            if (current.left != null) {
                queue.offer(current.left);  //如果当前节点的左节点不为空，则左节点入队列
            }
            if (current.right != null) {
                queue.offer(current.right); //如果当前节点的右节点不为空，则右节点入队列
            }
        }

        return arrayList;
        
    }
}