Add solution #204

Author: JoshCrozier

README.md

@@ -90,6 +90,7 @@
 191|[Number of 1 Bits](./0191-number-of-1-bits.js)|Easy|
 198|[House Robber](./0198-house-robber.js)|Medium|
 203|[Remove Linked List Elements](./0203-remove-linked-list-elements.js)|Easy|
+204|[Count Primes](./0204-count-primes.js)|Medium|
 206|[Reverse Linked List](./0206-reverse-linked-list.js)|Easy|
 213|[House Robber II](./0213-house-robber-ii.js)|Medium|
 214|[Shortest Palindrome](./0214-shortest-palindrome.js)|Hard|

solutions/0204-count-primes.js

@@ -0,0 +1,25 @@
+/**
+ * 204. Count Primes
+ * https://leetcode.com/problems/count-primes/
+ * Difficulty: Medium
+ *
+ * Given an integer n, return the number of prime numbers that are strictly less than n.
+ */
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var countPrimes = function(n) {
+  const nonPrimes = [];
+  let result = 0;
+  for (let i = 2; i < n; i++) {
+    if (!nonPrimes[i]) {
+      result++;
+      for (let j = 2; i * j < n; j++) {
+        nonPrimes[i * j] = 1;
+      }
+    }
+  }
+  return result;
+};