204. Count Primes

1.描述

Description:

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

2.分析

3.代码

int countPrimes(int n) {
    if (n <= 1) return 0;
    bool *isPrime = (bool*)malloc(n * sizeof(bool));
    for (unsigned int i = 0 ; i < n; ++i)
        isPrime[i] = true;
    int root = sqrt(n);
    for (unsigned int i = 2; i <= root; ++i) {
        if (isPrime[i]) {
            for (unsigned int j = i * i; j < n; j += i) {
                isPrime[j] = false;
            }
        }
    }
    int sum = 0;
    for (unsigned int i = 2; i < n; ++i) {
        if (isPrime[i]) ++sum;
    }
    free(isPrime);
    isPrime = NULL;
    return sum;
}