CodingMediumcod-g606

Subject CombinatoricsLevel Mid–Senior~20 minCommon in Algorithms & data structures interviewsIndustries Software development

Question

Given an integer n, return the number of structurally distinct binary search trees that store the values 1..n exactly once. n is between 0 and 19. (For n=0 there is one tree: the empty tree.)

Implement

num_trees(n: int) → int

Examples

in[3]out5

What a strong answer looks like

State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.

Learn the concepts

Vibe coding: describe the solution in plain language (or narrate it) and the coach grades your approach. Generating runnable code from your description is coming next.

Run or narrate your approach, then ask the coach.