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.

• The root of a binary tree.

Input: root = [-10, 9, 20, null, null, 15, 7]
Output: 42
Explanation: The max path is 15 -> 20 -> 7, sum = 42.