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32.3 按之字形顺序打印二叉树.md
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32.3 按之字形顺序打印二叉树.md
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# 32.3 按之字形顺序打印二叉树
[NowCoder](https://www.nowcoder.com/practice/91b69814117f4e8097390d107d2efbe0?tpId=13&tqId=11212&tPage=1&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking&from=cyc_github)
## 题目描述
请实现一个函数按照之字形打印二叉树,即第一行按照从左到右的顺序打印,第二层按照从右至左的顺序打印,第三行按照从左到右的顺序打印,其他行以此类推。
## 解题思路
```java
public ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
ArrayList<ArrayList<Integer>> ret = new ArrayList<>();
Queue<TreeNode> queue = new LinkedList<>();
queue.add(pRoot);
boolean reverse = false;
while (!queue.isEmpty()) {
ArrayList<Integer> list = new ArrayList<>();
int cnt = queue.size();
while (cnt-- > 0) {
TreeNode node = queue.poll();
if (node == null)
continue;
list.add(node.val);
queue.add(node.left);
queue.add(node.right);
}
if (reverse)
Collections.reverse(list);
reverse = !reverse;
if (list.size() != 0)
ret.add(list);
}
return ret;
}
```
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