204. Count Primes

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.

class Solution:
    def countPrimes(self, n):
        """
        :type n: int
        :rtype: int
        """
        prime = [1] * (n + 1)
        num = 0
        for i in range(2,n):
            if prime[i]:
                num += 1
                for j in range(2,n):
                    if i*j<=n:
                        prime[i*j] = 0
                    else:
                        break
        return num

经典的素数筛法