import java.util.ArrayList; import java.util.Deque; import java.util.LinkedList; /* * 从上往下打印出二叉树的每个节点，同层节点从左至右打印。 */ public class Solution8 { public class TreeNode { int val = 0; TreeNode left = null; TreeNode right = null; public TreeNode(int val) { this.val = val; } } public ArrayList PrintFromTopToBottom(TreeNode root) { ArrayList list = new ArrayList<>(); if (root == null) { return list; } Deque deque = new LinkedList<>(); deque.add(root); while (!deque.isEmpty()) { TreeNode p = deque.pop(); list.add(p.val); if(p.left!=null) deque.add(p.left); if(p.right!=null) deque.add(p.right); } return list; } }