Вариант 1

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean contains(TreeNode root, TreeNode node) {
        if (root == null) {
            return false;
        }
        if (root.val == node.val) {
            return true;
        }
        return contains(root.left, node) || contains(root.right, node);
    }
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (contains(root, p) && contains(root, q)) {
            TreeNode node = lowestCommonAncestor(root.left, p, q);
            if (node == null) {
                node = lowestCommonAncestor(root.right, p, q);
                if (node == null) {
                    node = root;
                }
            }
            return root;
        }
        return null;
    }
}