java编程题之从上往下打印出二叉树

import java.util.ArrayList;import java.util.Stack;/** *  * 剑指offer编程题（JAVA实现)――第22题：从上往下打印出二叉树 *  * 题目描述 * 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 Test22 { ArrayList<Integer> arrayList = new ArrayList<>(); // 每层依次入栈 Stack<TreeNode> stack1 = new Stack<>(); // 从stack1出栈的元素依次加入stack2，统一通过stack2找到他们的字节点并压入stack1 Stack<TreeNode> stack2 = new Stack<>(); public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) { if (root == null) { return arrayList;// 空则返回 } stack1.push(root); while (!stack1.isEmpty()) { while (!stack1.isEmpty()) { TreeNode tmp = stack1.pop(); arrayList.add(tmp.val); stack2.push(tmp); } while (!stack2.isEmpty()) { TreeNode tmp2 = stack2.pop(); // 从左到右打印，所以右子树先入栈 if (tmp2.right != null) { stack1.push(tmp2.right); } if (tmp2.left != null) { stack1.push(tmp2.left); } } } return arrayList; } public class TreeNode { int val = 0; TreeNode left = null; TreeNode right = null; public TreeNode(int val) { this.val = val; } }}//其他方法/**public class Solution { public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {  ArrayList<Integer> list = new ArrayList<Integer>();  if(root == null) return list;  Deque<TreeNode> deque = new LinkedList<TreeNode>();    deque.add(root);  while(!deque.isEmpty()){  TreeNode t = deque.pop();  list.add(t.val);  if(t.left != null) deque.add(t.left);  if(t.right != null) deque.add(t.right);  }  return list; }}*/