Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.
“The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        _______6______
       /              \
    ___2__          ___8__
   /      \        /      \
   0      _4       7       9
         /  \
         3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

Solution： 遞歸check

思路：
Time Complexity: O(N) Space Complexity: O(N) 遞歸緩存

Solution Code：

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root == null) return null;
        /* if necessary
        if(p == null || q == null) {
            if(p == null) return q;
            else return p;
        }
        */
        
        if(p.val < root.val && q.val < root.val) {
            return lowestCommonAncestor(root.left, p, q);
        }
        else if(p.val > root.val && q.val > root.val) {
            return lowestCommonAncestor(root.right, p, q);
        }
        // this node is the LCA
        else return root;
    }
}