DataStructures-Algorithms/Striver Sheet/Day-21/Kth Smallest Element in a BST.cpp at master · arjunkumar09/DataStructures-Algorithms · GitHub

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/*
Kth Smallest Element in a BST
=============================

Given the root of a binary search tree, and an integer k, return the kth (1-indexed) smallest element in the tree.

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

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

Constraints:
The number of nodes in the tree is n.
1 <= k <= n <= 104
0 <= Node.val <= 104

Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?
*/

/**
 * 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:
  int inorder(TreeNode *root, int k, int &val)
  {
    int ans = INT_MAX;

    if (root->left)
      ans = inorder(root->left, k, val);
    if (ans != INT_MAX)
      return ans;

    val++;
    if (val == k)
      return (root->val);

    if (root->right)
      ans = inorder(root->right, k, val);
    return ans;
  }

  int kthSmallest(TreeNode *root, int k)
  {
    int val = 0;
    return inorder(root, k, val);
  }
};