剑指offer--从上往下打印二叉树

/**
 * 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
 */
package javabasic.nowcoder;

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

import javax.management.Query;

class TreeNode27 {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode27(int val) {
        this.val = val;

    }

}
/*
 * 借组一个队列辅助层次遍历二叉树
 */
public class Main27 {

	public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
		ArrayList<Integer> arr = new ArrayList<Integer>();
		if(root==null) {
			return arr;
		}
		Queue<TreeNode> queue = (Queue<TreeNode>) new LinkedList();
		queue.offer(root);
		while(!queue.isEmpty()) {
			TreeNode temp = queue.poll();
			if(temp.left!=null) {
				queue.offer(temp.left);
			}
			if(temp.right!=null) {
				queue.offer(temp.right);
			}
			
			arr.add(temp.val);
		}
        return arr;
    }
	
}