root of a binary tree and an integer
true if the tree has a root-to-leaf path such that adding up all the values along the path equals
A leaf is a node with no children.
Input: root = [5,4,8,11,null,13,4,7,2,null,null,null,1], targetSum = 22 Output: true
Input: root = [1,2,3], targetSum = 5 Output: false
Input: root = [1,2], targetSum = 0 Output: false
-1000 <= Node.val <= 1000
-1000 <= targetSum <= 1000