leetcode204. Counting prime numbers-prime sieve

204. Counting prime numbers

Difficulty: simple

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

Example:

输入: 10
输出: 4
解释: 小于 10 的质数一共有 4 个, 它们是 2, 3, 5, 7 。

Solution

Upper prime number sieve (( a knowledge point water twice
is not a linear sieve... to be optimized

if n<2:
    return 0
p=[1]*n
s=0
for i in range(2,n):
    if p[i]==1:
        k=i
        s+=1
        while i*k<n:
            p[i*k]=0
            k+=1
return s