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Added to CountPrimesUsingSieve.py #697

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14 changes: 14 additions & 0 deletions CountPrimesUsingSieve.py
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# Python Sieve of Eratosthenes - https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
# The idea of the sieve of Eratosthene is to start with the list of all integer starting at 2 up to the limit we want to compute.
# A prime number is not the multiple of any number except 0, 1 and itself.
# So a way to find all primes up to the limit is to remove all multiples of the primes we currently know of starting at 2.
class Solution:
def countPrimes(self, n: int) -> int:
candidates = list(range(2,n))
for idx, num in enumerate(candidates):
if num is not None:
for multiple in range(idx+num, len(candidates), num):
candidates[multiple] = None
return sum(1 for i in candidates if i is not None)

# This algorithm is good if you want preprocessed data of prime numbers. #SharingKnowledge #Hacktoberfest2020