solve 204.count-primes

Author: andavid

vscode/204.count-primes.java

@@ -0,0 +1,48 @@
+/*
+ * @lc app=leetcode id=204 lang=java
+ *
+ * [204] Count Primes
+ *
+ * https://leetcode.com/problems/count-primes/description/
+ *
+ * algorithms
+ * Easy (28.26%)
+ * Total Accepted:    227K
+ * Total Submissions: 791.5K
+ * Testcase Example:  '10'
+ *
+ * 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.
+ *
+ *
+ */
+class Solution {
+  public int countPrimes(int n) {
+    boolean[] primes = new boolean[n];
+    Arrays.fill(primes, true);
+
+    for (int i = 2; i <= Math.sqrt(n-1); i++) {
+      if (primes[i]) {
+        for (int j = i*i; j < n; j += i) {
+          primes[j] = false;
+        }
+      }
+    }
+
+    int count = 0;
+    for (int i = 2; i < n; i++) {
+      if (primes[i]) {
+        count++;
+      }
+    }
+
+    return count;
+  }
+}
+