从上往下打印二叉树

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