Maximum Sum between Two Leaf Nodes in Binary Tree

Given a binary tree where each node contains an integer value. Find the maximum path sum between any two leaf nodes. The path must start at one leaf and end at another leaf. Implement a function to solve this problem using DFS and maintain a global maximum. Test cases: Input: [1, 2, 3, 4, 5, null, null], Output: 12 Input: [-10, 9, 20, null, null, 15, 7], Output: 42 Input: [2, -1, null], Output: 2 Input: [10, -9, 20, null, null, 15, 7], Output: 42 Input: [5, 4, 8, 11, null, 13, 4, 7, 2, null, null, null, null, 1], Output: 48 Example Input 1, 2, 3, 4, 5, null, null