Lowest common ancestor

如果兩個節點都在樹裡，那麼用分治法可以很快解決

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode A, TreeNode B) {
    if (root == null || root == A || root == B) {
        return root;
    }

    TreeNode left = lowestCommonAncestor(root.left, A, B);
    TreeNode right = lowestCommonAncestor(root.right, A, B);

    if (left != null && right != null) {
        return root;
    }
    if (left != null) {
        return left;
    }
    if (right != null) {
        return right;
    }
    return null;
}