class Solution {
public:
    // 二叉搜索树中序遍历是有序的
    int k,ans;
    int kthSmallest(TreeNode* root, int _k) {
        k=_k;
        dfs(root);
        return ans;
    }
    bool dfs(TreeNode* root)
    {
        if(!root)return false;
        if(dfs(root->left))return true;
        if(--k==0)
        {
            ans=root->val;
            return true;
        }
        return dfs(root->right);
    }
};