CodingMediumcod-g409

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

Question

Given an integer n (0 <= n <= 5e6), return the count of prime numbers strictly less than n. For example for n=10 the primes below it are 2,3,5,7 so the answer is 4. A sieve of Eratosthenes running in O(n log log n) is expected; trial-dividing each number is too slow at the upper bound.

Implement

count_primes(n: int) → int

Examples

in[10]out4

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.