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.

思路：

Solution (C++):

int countPrimes(int n) {
    int count = 0;
    vector<bool> isPrime(n, false);
    for (int i = 2; i < n; ++i) {
        isPrime[i] = true;
    }
    for (int i = 2; i*i < n; ++i) {
        if (!isPrime[i])  continue;
        for (int j = i * i; j < n; j += i) {
            isPrime[j] = false;
        }
    }
    for (int i = 2; i < n; ++i) {
        if (isPrime[i])  ++count;
    }
    return count;
}

性能：

Runtime: 208 ms  Memory Usage: 6.6 MB

思路：

Solution (C++):

性能：

Runtime: ms  Memory Usage: MB