Find the Longest Edge Path in a Binary Tree

### Problem Return the diameter of the binary tree rooted at root: the maximum number of edges on a path joining any two nodes. The path may pass through a node other than the root. TreeNode has val, left and right fields. ```cpp class Solution { public: int diameterOfBinaryTree(TreeNode* root); }; ``` ### Examples **Example 1:** ```text root = [9,5,12,3,null,null,15,1] Output: 5 Explanation: 1 -> 3 -> 5 -> 9 -> 12 -> 15 has five edges. ``` **Example 2:** ```text root = [4] Output: 0 Explanation: A singleton has no edge. ``` ### Constraints - The binary tree contains 1..10000 nodes, with values from -100 through 100. ### Clarifying Questions Count edges rather than nodes and allow the maximizing path to remain inside one subtree. ```hint Read the boundary condition Which two subtree measurements determine the best path whose highest node is the current node? ``` ### Contract considerations Count edges rather than nodes and allow the maximizing path to remain inside one subtree.