MEDIUM NC#57 Blind #69 Trees
230. Kth Smallest Element in a BST
📖 Problem
Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) in all the BST. It is guaranteed that there will always be a valid answer.
🧠 Visual Learning Aid
1 Model the input into the right structure
2 Choose the core technique and invariant
3 Execute step-by-step with a sample
4 Validate complexity and edge cases
JS/TS Refreshers
- •Array methods (`push`, `pop`, `shift`, `slice`)
- •Object/Map/Set usage patterns
- •Function parameter and return typing
- •Midpoint overflow-safe math
- •Loop invariants
- •Monotonic condition design
Logical Thinking Concepts
- •Define invariants before coding
- •Check edge cases first (`[]`, single element, duplicates)
- •Estimate time/space before implementation
- •Apply Binary Search reasoning pattern
💡 Approach
- → Use iterative inorder traversal with stack
- → Kth smallest element is the kth element in inorder traversal
- → Keep track of count while traversing
- → When count reaches k, we've found our answer
- → Time: O(h + k), Space: O(h) where h is tree height
🧭 Prerequisites
🛠️ Hints & Pitfalls
Hints
- •Use iterative inorder traversal with stack
- •Kth smallest element is the kth element in inorder traversal
- •Keep track of count while traversing
Common Pitfalls
- •When count reaches k, we've found our answer
- •Time: O(h + k), Space: O(h) where h is tree height
🧪 Test Cases
Test Case 1
Not run Input:
_kthSmallest(_bst1, 1); Expected:
1 Test Case 2
Not run Input:
_kthSmallest(_bst2, 3); Expected:
4 Test Case 3
Not run Input:
_kthSmallest(root: TreeNode | null, k: number); Expected:
Computed from hidden reference 📝 Code Editor
📤 Output