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.

思路：建一个[True]*n的数组result，result[0]=result[1]=False,从二开始遍历n，如果为True，那result[n*i]必不为素数，result[n*i]=False,返回reslut中True个数

代码:python3

class Solution:
    def countPrimes(self, n: int) -> int:
        if n < 2:
            return 0
        result = [True]*n
        result[0]=result[1]=False
        for i in range(2,n):
            if result[i] == True:
                for j in range(2,int(n/i)+1):
                    if i*j < n:
                        result[i*j]=False
        return sum(result)