Day 20 Kth Smallest Element in a BST.cpp

PROBLEM:


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

Note:
You may assume k is always valid, 1 ≤ k ≤ BST's total elements.

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?




SOLUTION:


/**
 * 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:
    
    void helper(TreeNode* root,int& k,int& ans)
    {   
        if(root==NULL || k==0)
            return;
        
        helper(root->left,k,ans);
        k--;
        if(k==0)
        {
            ans=root->val;
        }
        helper(root->right,k,ans);
    }
    
    int kthSmallest(TreeNode* root, int k) {
        
        int ans;
        helper(root,k,ans);
        
        
        return ans;
        
    }
};