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: [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. Note: The number of nodes in the given tree is between 1 and 10^4. -10^5 <= node.val <= 10^5