Dr. Rick Sun code
 public TreeNode LCA(TreeNode root, TreeNode a, TreeNode b) {
        if (root == null ||root == a || root == b) return root;// base case 1; case 3
        TreeNode left = LCA(root.left, a, b);
        TreeNode right = LCA(root.right, a, b); // recursion step 1
        if (left != null && right != null) return root;// recursion step 2 + 3; case 1
        return (left != null ? left : right);                                // case 2
 }