[Leetcode]Kth Smallest Element in a BST

//用search计算左子树的节点个数，加上根节点本身若为k则输出，否则

//（1）若k大于目前个数，则k-左子树节点个数，再计算右子树

//（2）若k小于目前个数，则直接计算左子树

/**

* Definition for a binary tree node.

* struct TreeNode {

* int val;

* TreeNode *left;

* TreeNode *right;

* TreeNode(int x) : val(x), left(NULL), right(NULL) {}

* };

*/

class Solution {

private:

int result=0;

int search(TreeNode* root)

{

if(root==NULL)

return 0;

return 1+search(root->left)+search(root->right);

}

public:

int kthSmallest(TreeNode* root, int k) {

int result;

int count=search(root->left)+1;

if(count==k)return root->val;

else if(count>k)kthSmallest(root->left,k);

else if(count<k)kthSmallest(root->right,k-count);

}

};