leetcode204. Count Primes

Description:

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

java

public class Solution {    public int countPrimes(int n) {        boolean[] a= new boolean[n];        for (int i=2;i*i<n;i++){            if(!a[i]){                for(int j=i;i*j<n;j++){                    a[i*j]=true;                }            }        }        int count=0;        for(int i=2;i<n;i++){            if(a[i]==false) ++count;        }        return count;    }}

python

def countPrimes(n):    if n<=1:        return 0    elif n==2:        return 1    a={}    for i in range(2,n):        a.setdefault(i,None)    for i in range(2,n):        temp=n/i        for k in range(temp+1):            if k>1:                if k*i in a:                    a.pop(k*i)    return len(a)