层序遍历：
102.二叉树的层序遍历
https://leetcode.cn/problems/binary-tree-level-order-traversal/
107.二叉树的层次遍历II
https://leetcode.cn/problems/binary-tree-level-order-traversal-ii/submissions/
199.二叉树的右视图
https://leetcode.cn/problems/binary-tree-right-side-view/
637.二叉树的层平均值
https://leetcode.cn/problems/average-of-levels-in-binary-tree/submissions/
429.N叉树的层序遍历
https://leetcode.cn/problems/n-ary-tree-level-order-traversal/submissions/
515.在每个树行中找最大值
https://leetcode.cn/problems/find-largest-value-in-each-tree-row/submissions/
116.填充每个节点的下一个右侧节点指针
https://leetcode.cn/problems/populating-next-right-pointers-in-each-node/submissions/
117.填充每个节点的下一个右侧节点指针II
https://leetcode.cn/problems/populating-next-right-pointers-in-each-node-ii/submissions/
104.二叉树的最大深度
https://leetcode.cn/problems/maximum-depth-of-binary-tree/submissions/
111.二叉树的最小深度
https://leetcode.cn/problems/minimum-depth-of-binary-tree/submissions/
