kthSmallestElementInBST.md

Kth Smallest Element in a BST

Question

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

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
        Stack<TreeNode> stack=new Stack<>();
        TreeNode curr=root;
        while(curr!=null || !stack.isEmpty()){
            while(curr!=null){
                stack.push(curr);
                curr=curr.left;
            }
            if(--k!=0)
                curr=stack.pop();
            else
                return stack.pop().val;
            curr=curr.right;
        }
        return 0;
    }
}