kth_smallest_element_in_bst.cpp

/*
230. Kth Smallest Element in a BST
Medium

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

 

Example 1:

Input: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1
Output: 3

Follow up:
What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently?
How would you optimize the kthSmallest routine?
*/

/**
 * 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 {
public:
    vector<int> inorder(TreeNode* root, vector<int>& result){
    if(root!=NULL){
        inorder(root->left, result);
        result.push_back(root->val);
        inorder(root->right, result);
    }
        return result;
    }
    
    int kthSmallest(TreeNode* root, int k) {
     vector<int> result;
     vector<int> arr = inorder(root, result);
    return arr[k-1];
    }
};