leetcode230.二叉搜索树中第k小的元素

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);
    }
};