solve problem Count Primes

Author: P-ppc

Diff for: README.md

@@ -190,6 +190,7 @@ All solutions will be accepted!
 |532|[K Diff Pairs In An Array](https://leetcode-cn.com/problems/k-diff-pairs-in-an-array/description/)|[java/py/js](./algorithms/KDiffPairsInAnArray)|Easy|
 |633|[Sum Of Square Numbers](https://leetcode-cn.com/problems/sum-of-square-numbers/description/)|[java/py/js](./algorithms/SumOfSquareNumbers)|Easy|
 |605|[Can Place Flowers](https://leetcode-cn.com/problems/can-place-flowers/description/)|[java/py/js](./algorithms/CanPlaceFlowers)|Easy|
+|204|[Count Primes](https://leetcode-cn.com/problems/count-primes/description/)|[java/py/js](./algorithms/CountPrimes)|Easy|
 
 # Database
 |#|Title|Solution|Difficulty|

Diff for: algorithms/CountPrimes/README.md

@@ -0,0 +1,3 @@
+# Count Primes
+This problem is easy to solve by Sieve of Eratosthenes
+The js solution will not accept on leetcode-cn but ok on leetcode

Diff for: algorithms/CountPrimes/Solution.java

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

Diff for: algorithms/CountPrimes/solution.js

@@ -0,0 +1,26 @@
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var countPrimes = function(n) {
+    let primes = new Array(Math.max(n, 2)),
+        count = 0
+    
+    primes.fill(true)
+    primes[0] = primes[1] = false
+    
+    for (let i = 2; i < n; i++) {
+        if (primes[i]) {
+            for (let j = i * 2; j < n; j += i) {
+                primes[j] = false
+            }
+        }
+    }
+    
+    for (let i = 2; i < n; i++) {
+        if (primes[i]) {
+            count++
+        }
+    }
+    return count
+};

Diff for: algorithms/CountPrimes/solution.py

@@ -0,0 +1,21 @@
+class Solution(object):
+    def countPrimes(self, n):
+        """
+        :type n: int
+        :rtype: int
+        """
+        prime_list = [True] * max(n, 2)
+        prime_list[0] = prime_list[1] = False
+        
+        for i in range(2, n):
+            if prime_list[i]:
+                j = i * 2
+                while j < n:
+                    prime_list[j] = False
+                    j += i
+        
+        count = 0
+        for i in range(2, n):
+            if prime_list[i]:
+                count += 1
+        return count