204. Count Primes

问题描述：

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.

解题思路：

首先对2～n的每一个数字进行判断：

从质数开始，将质数的倍数置为false

然后从2开始检查，注意题目中要求less than,所以条件为i<n

代码：

class Solution {
public:
    int countPrimes(int n) {
        vector<bool> isPrime(n+1, true);
        for(int i = 2; i*i <= n; i++){
            if(isPrime[i]){
                for(int j = i*2; j <= n; j += i){
                    isPrime[j] = false;
                }
            }
        }
        int ret = 0;
        for(int  i = 2; i < n; i++){
            if(isPrime[i]) ret++;
        }
        return ret;
    }
};