原題

https://leetcode.com/problems/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.

解法

假設0, 1, … n-1是質數, 初始化列表primes, 已知0和1不是質數, 設為0, 從最小的質數2開始, 2的所有倍數(2倍以上)都不是質數, 設為0. 最後統計質數列表的和.

程式碼:

class Solution:
    def countPrimes(self, n):
        """
        :type n: int
        :rtype: int
        """
        # base case
        if n < 3: return 0
        # suppose 0, 1, ...n-1 are primes
        primes = [1]*n
        # 0 and 1 are not primes
        primes[0] = primes[1] = 0
        for
 
 i in range(2, n):
            if primes[i] == True:
                for j in range(2, (n-1)//i+1):
                    primes[i*j] = 0            
                
        return sum(primes)