feat(0204): Count Primes

Author: chenzhao

0204. Count Primes.js

@@ -0,0 +1,26 @@
+// Count the number of prime numbers less than a non-negative number, n.
+
+// Example:
+
+// Input: 10
+// Output: 4
+
+// Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+const countPrimes = function(n) {
+  const notPrimes = Array(n).fill(false)
+  let res = 0
+  for (let i = 2; i < n; i++) {
+    if (notPrimes[i] === false) {
+      res++
+    }
+    for (let j = 2; i * j < n; j++) {
+      notPrimes[i * j] = true
+    }
+  }
+  return res
+}