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.

統計所有小於非負整數 n 的質數的數量。

示例:

輸入: 10
輸出: 4
解釋: 小於 10 的質數一共有 4 個, 它們是 2, 3, 5, 7 。

424ms

 1 class Solution {
 2     func countPrimes(_ n: Int) -> Int {
 
 3         if n < 2 {return 0}
 4         //篩選法
 5         var count = 0
 6         //創建具有默認值的數組
 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 }

204. 計數質數 | Count Primes