230. Kth Smallest Element in a BST

https://leetcode.com/problems/kth-smallest-element-in-a-bst/

Problem

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

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?

Constraints:

• The number of elements of the BST is between 1 to 10^4.

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

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:
    int kthSmallest(TreeNode* root, int k) {
        stack<TreeNode*> nodes;
        TreeNode* current = root;
        int kth_smallest = -1;
        while (!nodes.empty() || current != nullptr) {
            if (current != nullptr) {
                nodes.push(current);
                current = current->left;
            } else {
                kth_smallest = nodes.top()->val;
                current = nodes.top()->right;
                if (--k == 0) break;
                nodes.pop();
            }
        }
        return kth_smallest;
    }
};

• #tree

• #inorder