解题思路：

class Solution {
    List<Integer> list = new ArrayList<>();

    public int kthSmallest(TreeNode root, int k) {
        dfs(root);
        return list.get(k - 1);
    }

    public void dfs(TreeNode root) {
        if (root == null) return;
        dfs(root.left);
        list.add(root.val);
        dfs(root.right);
    }
}