中序遍历，第k次出现的数值就是结果 

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode() : val(0), left(nullptr), right(nullptr) {}*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}* };*/
class Solution {
private:int dfs(TreeNode* root,int& k){if(root==NULL)return -1;else{int result=dfs(root->left,k);if(result!=-1)return result;if(--k==0)return root->val;return dfs(root->right,k);}}
public:int kthSmallest(TreeNode* root, int k) {return dfs(root,k);}
};