Lowest Common Ancestor of a Binary Search Tree

Lowest Common Ancestor of a Binary Search Tree
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).”


Solution - c#:
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode LowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root==null)return null;
        if(IsFirstParentSecond(p,q))return p;
        if(IsFirstParentSecond(q,p))return q;
        if((p.val<=root.val && q.val>=root.val) || (q.val<=root.val && p.val>=root.val))return root;
        if(p.val>root.val)return LowestCommonAncestor(root.right,p,q);
        else return LowestCommonAncestor(root.left,p,q);
        
        
    }
    public bool IsFirstParentSecond(TreeNode first,TreeNode second)
    {
        if((first.left!=null && first.left==second) ||(first.right!=null && first.right==second))return true;
        return false;
    }
}