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;

}

Comments

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