204. Count Primes

 1 class Solution {
 2 public:
 3     int countPrimes(int n) {
 4         vector<int> nums(n, 0);
 5         int count = 0;
 6         if (n > 2) ++ count; // 统计2
 7         for (int i = 3; i * i <= n; i += 2) { // 忽略偶数
 8             if (nums[i] == 0) {
 9                 for (int j = i * i; j <= n; j += i) { // j = i * i
10                     nums[j] = 1;
11                 }
12             }
13         }
14         for (int i = 3;  i < nums.size(); i += 2) {
15             if (nums[i] == 0) ++ count;
16         }
17         return count;
18     }
19 };