2019-11-02 12:07:41 +08:00
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# 32.1 从上往下打印二叉树
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[NowCoder](https://www.nowcoder.com/practice/7fe2212963db4790b57431d9ed259701?tpId=13&tqId=11175&tPage=1&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking&from=cyc_github)
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## 题目描述
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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。
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例如，以下二叉树层次遍历的结果为：1,2,3,4,5,6,7
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2019-11-02 17:33:10 +08:00
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<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/d5e838cf-d8a2-49af-90df-1b2a714ee676.jpg" width="250"/> </div><br>
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2019-11-02 12:07:41 +08:00
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## 解题思路
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使用队列来进行层次遍历。
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不需要使用两个队列分别存储当前层的节点和下一层的节点，因为在开始遍历一层的节点时，当前队列中的节点数就是当前层的节点数，只要控制遍历这么多节点数，就能保证这次遍历的都是当前层的节点。
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```java
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public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
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Queue<TreeNode> queue = new LinkedList<>();
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ArrayList<Integer> ret = new ArrayList<>();
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queue.add(root);
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while (!queue.isEmpty()) {
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int cnt = queue.size();
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while (cnt-- > 0) {
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TreeNode t = queue.poll();
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if (t == null)
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continue;
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ret.add(t.val);
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queue.add(t.left);
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queue.add(t.right);
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}
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}
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return ret;
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}
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```
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2019-11-02 17:33:10 +08:00
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<div align="center"><img width="320px" src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/githubio/公众号二维码-2.png"></img></div>
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