@@ -1 +1,33 @@
-
+/**
+ *  Qusetion:
+ *    Count the number of prime numbers less than a non-negative number, n.
+ *  Url and solution:
+ *    https://leetcode.com/problems/count-primes/ 
+ * 
+ * /
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var countPrimes = function(n) {
+   var pHash = {};
+   var count = 0;
+   
+   for (var k = 2; k < n; k++) {
+      pHash[k] = true;
+   }
+   // Loop's ending condition is i * i < n instead of i < sqrt(n)
+   // to avoid repeatedly calling an expensive function sqrt().
+   for (var i = 2; i * i < n; i++) {
+      if (!pHash[i]) continue;
+      for (var j = i * i; j < n; j += i) {
+         pHash[j] = false;
+      }
+   }
+   
+   for (var l = 2; l < n; l++) {
+      if (pHash[l]) count++;
+   }
+   
+   return count;
+};