LeetCode-in-Cpp

230. Kth Smallest Element in a BST

Medium

Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes 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?

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 {
private:
    int k;
    int count = 0;
    int val;

public:
    int kthSmallest(TreeNode* root, int k) {
        this->k = k;
        calculate(root);
        return val;
    }

private:
    void calculate(TreeNode* node) {
        if (node->left == nullptr && node->right == nullptr) {
            count++;
            if (count == k) {
                val = node->val;
            }
            return;
        }
        if (node->left != nullptr) {
            calculate(node->left);
        }
        count++;
        if (count == k) {
            val = node->val;
            return;
        }
        if (node->right != nullptr) {
            calculate(node->right);
        }
    }
};

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