题目：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。
思路：
由题意可知，从上往下，从左往右就是层次遍历。
源码：GitHub源码
import java.util.*;
/**
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
*/
public class Solution {
public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
ArrayList<Integer>list = new ArrayList<>();
if(root == null) return list;
Deque<TreeNode> deque = new LinkedList<TreeNode>();
deque.add(root);
while(!deque.isEmpty()){
if(deque.peek().left != null){
deque.add(deque.peek().left);
}
if(deque.peek().right != null){
deque.add(deque.peek().right);
}
list.add(deque.pollFirst().val);
}
return list;
}
}
