层序遍历二叉树

package com.algorithm;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;

//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 TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;
    }
}
*/
public class PrintBinaryTree {
    //思路 ，队列
     public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
         ArrayList<Integer> ls = new ArrayList<Integer>();
         if(root == null)
             return ls;
         Queue<TreeNode> queue = new LinkedList<TreeNode>();
         queue.offer(root);//入队
         while(!queue.isEmpty()){
             TreeNode tn = queue.poll();//出对
             ls.add(tn.val);//保存值
             if(tn.left != null)
                 queue.offer(tn.left);//入队
             if(tn.right != null)
                 queue.offer(tn.right);//出对
         }
         return ls;
      }
}