leetcode 104 maximum-depth-of-binary-tree
https://leetcode.cn/problems/maximum-depth-of-binary-tree/submissions/https://www.bilibili.com/video/BV11Y4y1q7YA?p=27Giventherootofabinarytree,returnitsmaximumdepth.Abinarytree'smaximumdepth isthenumberofnodesalongthelongest