Maximum Level Sum of a Binary Tree

Given the `root` of a binary tree, the level of its root is `1`, the level of its children is `2`, and so on. Return the **smallest** level `x` such that the sum of all the values of nodes at level `x` is **maximal**.   **Example 1:** **Input:** root = [1,7,0,7,-8,null,null] **Output:** 2 **Explanation: ** Level 1 sum = 1. Level 2 sum = 7 + 0 = 7. Level 3 sum = 7 + -8 = -1. So we return the level with the maximum sum which is level 2. ``` **Example 2:** **Input:** root = [989,null,10250,98693,-89388,null,null,null,-32127] **Output:** 2 ```   **Constraints:** - The number of nodes in the tree is in the range `[1, 104]`. - `-105 <= Node.val <= 105`