Implement prime counting function in countPrimes.java

Author: ArsiyaTasleem90250

countPrimes.java

@@ -0,0 +1,24 @@
+class Solution {
+    public int countPrimes(int n) {
+        if(n<2){
+            return 0 ;
+        }
+        boolean isPrime[] = new boolean[n+1];
+        Arrays.fill(isPrime , true);  
+        isPrime[0] = false;
+        isPrime[1] = false;
+        for(int i=2 ; i*i<=n ; i++){
+            for(int j = 2*i;j<=n;j+=i){
+                isPrime[j] = false;
+            }
+        }
+        int count=0;
+        for(int i=0;i<n;i++){
+            if(isPrime[i]==true){
+                count++;
+            }
+        }
+        return count;
+    }
+}
+