LeetCode-in-All

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.
 * class TreeNode {
 *     public $val = null;
 *     public $left = null;
 *     public $right = null;
 *     function __construct($val = 0, $left = null, $right = null) {
 *         $this->val = $val;
 *         $this->left = $left;
 *         $this->right = $right;
 *     }
 * }
 */
class Solution {
    private $k;
    private $count = 0;
    private $val;

    /**
     * @param TreeNode $root
     * @param Integer $k
     * @return Integer
     */
    public function kthSmallest($root, $k) {
        $this->k = $k;
        $this->calculate($root);
        return $this->val;
    }

    private function calculate($node) {
        if ($node->left == null && $node->right == null) {
            $this->count++;
            if ($this->count == $this->k) {
                $this->val = $node->val;
            }
            return;
        }
        if ($node->left != null) {
            $this->calculate($node->left);
        }
        $this->count++;
        if ($this->count == $this->k) {
            $this->val = $node->val;
            return;
        }
        if ($node->right != null) {
            $this->calculate($node->right);
        }
    }
}

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