Added 204_count_primes_leetcode_seive_of_erathosthenes.cpp

C++/204_count_primes_leetcode_seive_of_erathosthenes.cpp

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+// Problem Link : https://leetcode.com/problems/count-primes/
+
+class Solution {
+public:
+    int countPrimes(int n) {
+        long long int i, j, ans = 0;
+        vector <bool> isprime(n + 1, true); // true if prime otherwise false
+        isprime[0] = isprime[1] = false; // 0, 1 are not prime numbers
+
+//         Using seive of erathosthenes to find the prime number numbers in the range from 1 to n
+        for (i = 2; i * i <= n; i++)
+        {
+            if (isprime[i])
+            {
+                for (j = i * i; j <= n; j = j + i) // once i * i >= n, then j = j + i always > n, so use of checking those index / numbers
+                {
+                    isprime[j] = false;
+                }
+            }
+        }
+
+        for (i = 1; i < n; i++) // Checking which all are primes numbers till n (n excluded)
+        {
+            if (isprime[i])
+                ans++;
+        }
+
+        return ans;
+    }
+};