二叉搜索树中第K小元素

class Solution {
    List<Integer> list = new ArrayList<>();
    public void dfs(TreeNode root){
        if(root == null) return;
        dfs(root.left);
        list.add(root.val);
        dfs(root.right);
    }
    public int kthSmallest(TreeNode root, int k){
        list = new ArrayList<>();
        dfs(root);
        for(int i = 0; i < list.size(); i++){
            if(i == k - 1)
                return list.get(i);
        }
        return -1;
    }
}