A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence at most once. Note that the path does not need to pass through the root.
The path sum of a path is the sum of the node's values in the path.
Given the root of a binary tree, return the maximum path sum of any non-empty path.
[1, 3 * 10^4]-1000 <= Node.val <= 1000root = [1,2,3]6root = [-10,9,20,null,null,15,7]42