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
SOLUTION:
import math
class Solution:
def countPrimes(self, n: int) -> int:
if n <= 2:
return 0
primes = [1] * n
primes[0] = 0
primes[1] = 0
numprimes = n - 2
l = int(math.sqrt(n)) + 1
for i in range(2, l):
if primes[i] == 0:
continue
for j in range(i, 1 + n // i):
if i * j < n and primes[i * j] == 1:
primes[i * j] = 0
numprimes -= 1
return numprimes
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