Primes

ardallie · ardallie · commit 818735d47702 · 2021-08-17T02:06:42.000+01:00
diff --git a/src/medium/Primes.java b/src/medium/Primes.java
@@ -0,0 +1,62 @@
+package medium;
+
+/**
+ * Have the function Primes(num) take the num parameter being passed
+ * and return the string true if the parameter is a prime number,
+ * otherwise return the string false. The range will be between 1 and 2^16.
+ */
+public class Primes {
+
+  private static boolean isPrime(int num) {
+    // prime numbers are greater than 1
+    if (num <= 1) {
+      return false;
+    }
+    // 2 is an only even prime number
+    // http://mathworld.wolfram.com/EvenPrime.html
+    if (num == 2) {
+      return true;
+    }
+    // eliminate all even numbers to reduce time complexity
+    // (cannot be a prime number if is divisible by 2)
+    if (num % 2 == 0) {
+      return false;
+    }
+    // no need to check the entire range from 0 to num
+    // (square root of num) + 1 is the sufficient upper limit
+    // which greatly reduces time complexity
+    double upper = Math.sqrt(num) + 1;
+    // since the even numbers are eliminated, we can check the odd numbers only
+    for (int i = 3; i < upper; i += 2) {
+      if (num % i == 0) {
+        // not a prime number
+        return false;
+      }
+    }
+    // must be a prime number!
+    return true;
+  }
+
+  /**
+   * Primes function.
+   *
+   * @param num input number
+   * @return "true" if is a prime number
+   */
+  private static String primes(int num) {
+    return isPrime(num) ? "true" : "false";
+  }
+
+  /**
+   * Entry point.
+   *
+   * @param args command line arguments
+   */
+  public static void main(String[] args) {
+    String result1 = primes(195);
+    System.out.println(result1);
+    String result2 = primes(197);
+    System.out.println(result2);
+  }
+
+}
