Java 素数 prime numbers-LeetCode 204

Description:

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

click to show more hints.

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

求n以内的所有素数，以前看过的一道题目，通过将所有非素数标记出来，再找出素数，代码如下：

 1     public int countPrimes(int n) {
 2         if (n == 0 || n == 1 || n == 2)
 3             return 0;
 4         int[] flag = new int[n];
 5         for (int i = 2; i < Math.sqrt(flag.length); i++) {
 6             if (flag[i] == 0)
 7                 for (int j = i; i * j < flag.length; j++) {
 8                     flag[i * j] = 1;
 9                 }
10         }
11         int count = 0;
12         for (int i = 2; i < flag.length; i++)
13             count += flag[i];
14         BitSet bs = new BitSet();
15         bs.ne
16         return flag.length - count - 2;
17     }

值得注意的是外循环只需要到根号n即可，因为内循环是从i*j即i平方开始的。在LeetCode如果没加根号会产生溢出