Add C++ implementation

Signed-off-by: begeekmyfriend <[email protected]>

Author: begeekmyfriend

0204_count_primes/count_primes.c

@@ -1,12 +1,14 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdbool.h>
-#include <string.h>
 
-static int countPrimes(int n)
+
+int countPrimes(int n)
 {
-    if (n < 3) return 0;
-    
+    if (n < 3) {
+        return 0;
+    }
+
     int i, j;
     bool *marked = malloc(n);
     memset(marked, false, n);

0204_count_primes/count_primes.cc

@@ -0,0 +1,26 @@
+#include <stdc++.h>
+
+using namespace std;
+
+class Solution {
+public:
+    int countPrimes(int n) {
+        if (n < 3) {
+            return 0;
+        }
+
+        vector<bool> marked(n);
+        int count = n >> 1;
+        for (int i = 3; i * i <= n; i += 2) {
+            if (!marked[i]) {
+                for (int j = i * i; j < n; j += (i << 1)) {
+                    if (!marked[j]) {
+                        marked[j] = true;
+                        --count;
+                    }
+                }
+            }
+        }
+        return count;
+    }
+};