leet49_二叉树中序遍历

public List inorderTraversal(TreeNode root) {
    List res = new ArrayList();
    inorder(root, res);
    return res;
}

public void inorder(TreeNode root, List res) {
    if (root == null) {
        return;
    }
    inorder(root.left, res);
    res.add(root.val);
    inorder(root.right, res);
}