Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes `p` and `q` as the lowest node in `T` that has both `p` and `q` as descendants (where we allow a node to be a descendant of itself).”

Example 1:

    Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
    Output: 6
    Explanation: The LCA of nodes 2 and 8 is 6.

Example 2:

    Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
    Output: 2
    Explanation: The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

Example 3:

    Input: root = [2,1], p = 2, q = 1
    Output: 2

Constraints:

- The number of nodes in the tree is in the range `[2, 105]`.
- `-109 <= Node.val <= 109`
- All `Node.val` are unique.
- `p != q`
- `p` and `q` will exist in the BST.