Java 素數 prime numbers-LeetCode 204

標籤：

Description:

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

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

 

求n以內的所有素數，以前看過的一道題目，通過將所有非素數標記出來，再找出素數，代碼如下：

 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.ne16         return flag.length - count - 2;17     }

值得注意的是外迴圈只需要到根號n即可，因為內迴圈是從i*j即i平方開始的。在LeetCode如果沒加根號會產生溢出

 

Java 素數 prime numbers-LeetCode 204