22、从上往下打印二叉树

public class Solution {
public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
ArrayList<Integer> list = new ArrayList<>();
ArrayList<TreeNode> queue = new ArrayList<>();
if (root == null) {
return list;
}
//根节点入队
queue.add(root);
while (queue.size() != 0) {
//节点出队并保存出队的节点
TreeNode temp = queue.remove(0);
list.add(temp.val);
//左节点入队
if (temp.left != null){
queue.add(temp.left);
}
//右节点入队
if (temp.right != null) {
queue.add(temp.right);
}
}
return list;
}
}