Merge pull request #1590 from raj-hero/master

Created leetcode-Count_Primes.cpp

Program's_Contributed_By_Contributors/C++/leetcode-Count_Primes.cpp

@@ -0,0 +1,24 @@
+// question:https://leetcode.com/problems/count-primes/submissions/812562322/
+class Solution {
+public:
+    int countPrimes(int n) 
+    {
+      // sieve algorithm
+        vector<bool> primes(n+1,true);
+        primes[0]=primes[1]=false;
+        for(int i=2;i<=n;++i){
+            if(primes[i]==true){
+                for(int j=2*i;j<=n;j+=i){
+                    primes[j]=false;
+                }
+            }
+        }
+     // queries
+        int ct=0;
+        for(int i=0;i<n;++i)
+        {
+            if(primes[i]==true) ++ct;
+        }
+        return ct;
+    }
+};