-
Notifications
You must be signed in to change notification settings - Fork 22
Expand file tree
/
Copy path204.count-primes.cpp
More file actions
80 lines (71 loc) · 1.53 KB
/
204.count-primes.cpp
File metadata and controls
80 lines (71 loc) · 1.53 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
// Tag: Array, Math, Enumeration, Number Theory
// Time: O(NlogN)
// Space: O(N)
// Ref: -
// Note: -
// Given an integer n, return the number of prime numbers that are strictly less than n.
//
// Example 1:
//
// Input: n = 10
// Output: 4
// Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.
//
// Example 2:
//
// Input: n = 0
// Output: 0
//
// Example 3:
//
// Input: n = 1
// Output: 0
//
//
// Constraints:
//
// 0 <= n <= 5 * 106
//
//
class Solution {
public:
int countPrimes(int n) {
if (n <= 2) {
return 0;
}
vector<bool> prime(n, true);
int count = n - 2;
for (int i = 2; i < sqrt(n) + 1; i++) {
if (prime[i]) {
for (int j = i * i; j < n; j += i) {
if (prime[j]) {
prime[j] = false;
count--;
}
}
}
}
return count;
}
};
class Solution {
public:
int countPrimes(int n) {
if (n <= 2) {
return 0;
}
vector<bool> prime(n, true);
int count = n / 2 - 1; // 去掉1
for (int i = 3; i < sqrt(n) + 1; i+=2) {
if (prime[i]) {
for (int j = i * i; j < n; j += 2 * i) {
if (prime[j]) {
prime[j] = false;
count--;
}
}
}
}
return count + 1; // 加上2
}
};