204. Count Primes

Problem

Count the number of prime numbers less than a non-negative number, n.

Example:

Input: 10
Output: 4
Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.

Problem Summary

Count the number of primes less than the non-negative integer n.

Solution Approach

Given a number n, output the total count of all prime numbers less than n. Easy problem.

Code


package leetcode

func countPrimes(n int) int {
	isNotPrime := make([]bool, n)
	for i := 2; i*i < n; i++ {
		if isNotPrime[i] {
			continue
		}
		for j := i * i; j < n; j = j + i {
			isNotPrime[j] = true
		}
	}
	count := 0
	for i := 2; i < n; i++ {
		if !isNotPrime[i] {
			count++
		}
	}
	return count
}