Count Primes

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

 1 public int countPrimes(int n) {
 2 
 3     ArrayList<Integer> primes = new ArrayList<Integer>();
 4 
 5     if (n <= 3) return Math.max(0, n - 1);
 6 
 7     primes.add(2);
 8     primes.add(3);
 9 
10     for (int i = 4; i <= n; i++) {
11         boolean isPrime = true;
12         for (int p : primes) {
13             int m = i % p;
14             if (m == 0) {
15                 isPrime = false;
16                 break;
17             }
18         }
19 
20         if (isPrime) {
21             primes.add(i);
22         }
23     }
24 
25     return primes.size();
26 }

 

 1 class Solution {
 2     public int countPrimes(int n) {
 3         boolean[] notPrime = new boolean[n];
 4         int count = 0;
 5         for (int i = 2; i < n; i++) {
 6             if (notPrime[i] == false) {
 7                 count++;
 8                 for (int j = 2; i * j < n; j++) {
 9                     notPrime[i * j] = true;
10                 }
11             }
12         }
13         return count;
14     }
15 }

 

转载于:https://www.cnblogs.com/beiyeqingteng/p/5722653.html