Given the `root` of a binary tree and an integer `targetSum`, return `true` if the tree has a root-to-leaf path such that adding up all the values along the path equals `targetSum`.

A leaf is a node with no children.

Example 1:

    Input: root = [5,4,8,11,null,13,4,7,2,null,null,null,1], targetSum = 22
    Output: true
    Explanation: The root-to-leaf path with the target sum is shown.

Example 2:

    Input: root = [1,2,3], targetSum = 5
    Output: false
    Explanation: There two root-to-leaf paths in the tree:
    (1 --> 2): The sum is 3.
    (1 --> 3): The sum is 4.
    There is no root-to-leaf path with sum = 5.

Example 3:

    Input: root = [], targetSum = 0
    Output: false
    Explanation: Since the tree is empty, there are no root-to-leaf paths.

Constraints:

- The number of nodes in the tree is in the range `[0, 5000]`.
- `-1000 <= Node.val <= 1000`
- `-1000 <= targetSum <= 1000`