# \[102]\[中等] 二叉树的层序遍历

## 题目描述

[102. 二叉树的层序遍历](https://leetcode-cn.com/problems/binary-tree-level-order-traversal/) [107. 二叉树的层次遍历 II](https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/) [429. N叉树的层序遍历](https://leetcode-cn.com/problems/n-ary-tree-level-order-traversal/) [637. 二叉树的层平均值](https://leetcode-cn.com/problems/average-of-levels-in-binary-tree/) [103. 二叉树的锯齿形层次遍历](https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/) [993. 二叉树的堂兄弟节点](https://leetcode-cn.com/problems/cousins-in-binary-tree/) [剑指 Offer 32 - I. 从上到下打印二叉树](https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-lcof/) [剑指 Offer 32 - II. 从上到下打印二叉树 II](https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof/) [剑指 Offer 32 - III. 从上到下打印二叉树 III](https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-iii-lcof/submissions/)

给你一个二叉树，请你返回其按 层序遍历 得到的节点值。 （即逐层地，从左到右访问所有节点）。

示例：

```
二叉树：[3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
```

返回其层次遍历结果：

```
[
  [3],
  [9,20],
  [15,7]
]
```

## 解题思路

使用队列. 按层进行循环, 借助的是记录当前层中节点的数量`size`, 对队列进行`size`次数的`pop`. 此时队列中的结点都是下一层的结点, 队列的长度就是下一层结点的数量. 向下循环重复之前的操作.

```python
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if root is None:
            return []

        res, queue = [], collections.deque()
        queue.append(root)
        while queue:
            nums = len(queue)
            layer = []
            for _ in range(nums):
                node = queue.popleft()
                layer.append(node.val)
                if node.left is not None:
                    queue.append(node.left)
                if node.right is not None:
                    queue.append(node.right)
            res.append(layer)
        return res
```

## 相关题目

[107. 二叉树的层次遍历 II](https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/) [429. N叉树的层序遍历](https://leetcode-cn.com/problems/n-ary-tree-level-order-traversal/) [637. 二叉树的层平均值](https://leetcode-cn.com/problems/average-of-levels-in-binary-tree/) [103. 二叉树的锯齿形层次遍历](https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/) [993. 二叉树的堂兄弟节点](https://leetcode-cn.com/problems/cousins-in-binary-tree/)