202 筛选质数

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<br>找出小于 n的质数的个数<br>注意2 就是第一个质数<br><br>class Solution { public:     int countPrimes(int n) {         vector<bool> num(n,true);         int res =0;         for(int i = 2 ;i<n ; i++){             if(num[i]){                 res++;                 for(int j=2 ;j*i<n;j++){                     num[j*i] = false;                 }             }         }         return res;     } };