LeetCode

Description:

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

素数筛法。居然还没有忘掉，难得啊。

class Solution {public:    int countPrimes(int n) {        vector<int> prime;        vector<bool> vis(n + 1, false);        for (int i = 2; i < n; ++i) {            if (vis[i]) continue;            vis[i] = true;            prime.push_back(i);            for (long long j = (long long)i * i; j < n; j += i) {                vis[j] = true;            }        }        return prime.size();    }};