题目：

题解：

func countPrimes(n int) int {
    primes := []int{}
    isPrime := make([]bool, n)
    for i := range isPrime {
        isPrime[i] = true
    }
    for i := 2; i < n; i++ {
        if isPrime[i] {
            primes = append(primes, i)
        }
        for _, p := range primes {
            if i*p >= n {
                break
            }
            isPrime[i*p] = false
            if i%p == 0 {
                break
            }
        }
    }
    return len(primes)
}