124. Binary Tree Maximum Path Sum
https://github.com/tongzhang1994/Facebook-Interview-Coding/blob/master/Longest%20Path%20in%20Binary%20Tree.java
https://www.youtube.com/watch?v=9ZNky1wqNUw&t=446s
687。543
原文：https://www.cnblogs.com/tobeabetterpig/p/9454937.html
