1324. The number of prime numbers

1324. The number of prime numbers

Chinese English

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

Sample

Sample 1

输入: n = 2
输出: 0

Sample 2

输入: n = 4
输出: 2
解析：2, 3 是素数

class Solution:
     '' '
     General idea:
     1. Initialization list [ 0 , 0 , 0 , 0 , 0 ], outer loop n, inner loop range (i * 2, n, i ), set the value to 1,
     2 . In the end [ 0 , 1 , 2 , 3 ] all of these default to 0, so you need to subtract the two values ​​of 0, 1 that are not prime numbers, which is the result. The ones that are not prime numbers are assigned a value of 1. 

    '' '
     def countPrimes (self, n): 
        l = [ 0 ] * n 
        ## At the beginning, set the prime number to 1 
        for   j in range ( 2 , n):
             if l [j]! = 1:
                 for z in range (j * 2 , n, j): # It can't be counted by itself, such as 5,7, can only take 10,14 as non-prime numbers, increasing by j. Until n. 
                    l [z] = 1 
        return l.count ( 0 ) -2