From 894a441c284a672fd1d3b02f43e0cf3d7a032f22 Mon Sep 17 00:00:00 2001
From: Khush3211 <34205476+khush3211@users.noreply.github.com>
Date: Sat, 17 Oct 2020 17:23:39 +0530
Subject: [PATCH] Added to CountPrimesUsingSieve.py

---
 CountPrimesUsingSieve.py | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
 create mode 100644 CountPrimesUsingSieve.py

diff --git a/CountPrimesUsingSieve.py b/CountPrimesUsingSieve.py
new file mode 100644
index 0000000..a0151df
--- /dev/null
+++ b/CountPrimesUsingSieve.py
@@ -0,0 +1,14 @@
+# 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