Count the number of prime numbers less than a non-negative number, n.
Example:
Input: 10 Output: 4 Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.
统计所有小于非负整数 n 的质数的数量。
示例:
输入: 10 输出: 4 解释: 小于 10 的质数一共有 4 个, 它们是 2, 3, 5, 7 。
424ms
1 class Solution {
2 func countPrimes(_ n: Int) -> Int {
3 if n < 2 {return 0}
4 //筛选法
5 var count = 0
6 //创建具有默认值的数组
7 var flag = Array(repeating: false, count: n)
8 var val:Int = Int()
9 for i in 2..<n
10 {
11 if flag[i] == false
12 {
13 count += 1
14 var j:Int = 1
15 val = j*i
16 while(true)
17 {
18 j += 1
19 val = j*i
20 if(val < n)
21 {
22 flag[val] = true
23 }
24 else
25 {
26 break
27 }
28 }
29 }
30 }
31 return count
32 }
33 }
84ms
1 class Solution {
2 func countPrimes(_ n: Int) -> Int {
3 guard n > 2 else { return 0 }
4 var sieve = [Bool](repeating: true, count: n)
5 var count = n / 2
6 var i = 3
7
8 while i * i < n {
9 if sieve[i] {
10 var j = i * i
11
12 while j < n {
13 if sieve[j] {
14 count -= 1
15 sieve[j] = false
16 }
17 j += 2 * i
18 }
19 }
20 i += 2
21 }
22
23 return count
24 }
25 }
112ms
1 class Solution {
2 func countPrimes(_ n: Int) -> Int {
3 guard n >= 3 else { return 0 }
4 var primes = [Bool](repeating: true, count: n)
5 var count = n/2
6 var i = 3
7 while i <= Int(Double(n).squareRoot()) {
8 if primes[i] {
9 var j = i
10 while i*j < n {
11 if primes[i*j] {
12 count -= 1
13 primes[i*j] = false
14 }
15 j += 2
16 }
17 }
18 i += 2
19 }
20 return count
21 }
22 }
原文地址：https://www.cnblogs.com/strengthen/p/9745292.html
时间： 2024-08-12 16:09:01