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;
 }
}
*/