MAY-20 2020 Challenge

Kth Smallest Element in a BST

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 in Java :

class Solution {public int kthSmallest(TreeNode root, int k) {

    Stack<TreeNode> stack = new Stack<TreeNode>();

    TreeNode p = root;

    while(p!=null){

        stack.push(p);

        p=p.left;

    }

    int i=0;

    while(!stack.isEmpty()){

        TreeNode t = stack.pop();

        i++;

        if(i==k)

            return t.val;

        TreeNode r = t.right;

        while(r!=null){

            stack.push(r);

            r=r.left;

        }

    }

    return -1;

}

    }