Kth Smallest Element in a BST LeetCode Solution

Problem – Kth Smallest Element in a BST

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?

Kth Smallest Element in a BST LeetCode Solution in Java

def findNode(node, res):
            if len(res) > 1:
                return

            if node.left:
                findNode(node.left, res)

            res[0] -= 1
            if res[0] == 0:
                res.append(node.val)
                return
            
            if node.right:
                findNode(node.right, res)
                
        res = [k]
        findNode(root, res)
        return res[1]

Kth Smallest Element in a BST LeetCode Solution in Python

def kthSmallest(self, root, k):
    self.k = k
    self.res = None
    self.helper(root)
    return self.res

def helper(self, node):
    if not node:
        return
    self.helper(node.left)
    self.k -= 1
    if self.k == 0:
        self.res = node.val
        return
    self.helper(node.right)

Kth Smallest Element in a BST LeetCode Solution in c++

int kthSmallest(TreeNode* root, int k) {
    return find(root, k);
}
int find(TreeNode* root, int& k) {
    if (root) {
        int x = find(root->left, k);
        return !k ? x : !--k ? root->val : find(root->right, k);
    }
}
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