## topic:

Given a binary search tree, please find the k-th smallest node in it.

Input: {5,3,7,2,4,6,8},3

Return value: {4}

Description: The value of the third small node is 4 in order of the numerical value of the node

## Ideas:

- First of all, the binary search tree is characterized by small left and large right, so if it is a middle-order traversal, it is exactly an ascending sequence
- So we can count at the same time on the basis of in-order traversal, Kth smallest = Kth from smallest to largest

## Code:

```
/*
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
*/
public class Solution {
private int count = 0;
TreeNode KthNode(TreeNode pRoot, int k) {
if(pRoot == null || k <= 0) return null;
return dfs(pRoot, k);
}
private TreeNode dfs(TreeNode root, int k){
if(root==null) return null;
TreeNode left = dfs(root.left, k);
if (left!=null) return left;
count++;
if(count==k) return root;
return dfs(root.right, k);
}
}
```