# Kth Smallest Element in a BST in Python

Suppose we have a binary search tree. We have to find the Kth smallest element in that BST. So if the tree is like −

So if we want to find 3rd smallest element, then k = 3, and result will be 7.

To solve this, we will follow these steps −

• create one empty list called nodes

• call solve(root, nodes)

• return k – 1th element of nodes

• the solve method is created, this takes root and nodes array, this will work as follows −

• if root is null, then return

• solve(left of root, nodes)

• add value of root into the nodes array

• solve(right of root, nodes)

## Example

Let us see the following implementation to get better understanding −

class Solution(object):
def kthSmallest(self, root, k):
"""
:type root: TreeNode
:type k: int
:rtype: int
"""
nodes = []
self.solve(root,nodes)
return nodes[k-1]
def solve(self, root,nodes):
if root == None:
return
self.solve(root.left,nodes)
nodes.append(root.val)
self.solve(root.right,nodes)

## Input

[10,5,15,2,7,13]

## Output

7
Published on 05-Feb-2020 10:14:44