LeetCode 204. Count Primes

Count Primes

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题目描述:

Description:

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

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题目思路:

题目给出一个数字n，求出小于n的素数的个数。

打表晒素数，统计素数的个数返回即可。

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题目代码:

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