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Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST. 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: 2 is smaller than the root 6 and 8 is larger, so they sit in different subtrees of 6. The lowest node that has both below it is 6.
Example 2
Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Explanation: 4 is in the right subtree of 2, and a node counts as its own descendant, so the lowest common ancestor is 2 itself.
Example 3
Input: root = [2,1], p = 2, q = 1
Output: 2
Explanation: 1 is the left child of 2, so the lowest common ancestor is 2.
In a BST the LCA is the first node where p and q fall on opposite sides (or one equals the node). Walk down: go left if both are smaller, right if both are larger, else you're at the split point.
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