From bf26fc664da746955a4f28a40ea9059366d0b6e2 Mon Sep 17 00:00:00 2001
From: Rodney Shaghoulian <9rodney@gmail.com>
Date: Mon, 19 Feb 2018 21:59:54 -0800
Subject: [PATCH] More efficient isPrime() method

---
 .../Solution.java                             |  6 ++++-
 .../Time Complexity - Primality/Solution.java |  8 ++++++-
 Java/Advanced/Prime Checker/Solution.java     |  8 +++++--
 .../Java Date and Time/Solution.java          | 22 +++++++++++--------
 4 files changed, 31 insertions(+), 13 deletions(-)

diff --git a/30 Days of Code/Day 25 - Running Time and Complexity/Solution.java b/30 Days of Code/Day 25 - Running Time and Complexity/Solution.java
index ac2cb23..49e382f 100644
--- a/30 Days of Code/Day 25 - Running Time and Complexity/Solution.java	
+++ b/30 Days of Code/Day 25 - Running Time and Complexity/Solution.java	
@@ -18,9 +18,13 @@ public class Solution {
     private static boolean isPrime(int n) {
         if (n < 2) {
             return false;
+        } else if (n == 2) {     // account for even numbers now, so that we can do i+=2 in loop below
+            return true;
+        } else if (n % 2 == 0) { // account for even numbers now, so that we can do i+=2 in loop below
+            return false;
         }
         int sqrt = (int) Math.sqrt(n);
-        for (int i = 2; i <= sqrt; i++) {
+        for (int i = 3; i <= sqrt; i += 2) { // skips even numbers for faster results
             if (n % i == 0) {
                 return false;
             }
diff --git a/Cracking the Coding Interview/Techniques, Concepts/Time Complexity - Primality/Solution.java b/Cracking the Coding Interview/Techniques, Concepts/Time Complexity - Primality/Solution.java
index 3eac536..f30680b 100644
--- a/Cracking the Coding Interview/Techniques, Concepts/Time Complexity - Primality/Solution.java	
+++ b/Cracking the Coding Interview/Techniques, Concepts/Time Complexity - Primality/Solution.java	
@@ -4,6 +4,8 @@
 
 import java.util.Scanner;
 
+//  Time Complexity: O(n^(1/2)) for each
+// Space Complexity: O(1)
 public class Solution {
     public static void main(String[] args) {
         Scanner scan = new Scanner(System.in);
@@ -18,9 +20,13 @@ public class Solution {
     public static boolean isPrime(int n) {
         if (n < 2) {
             return false;
+        } else if (n == 2) {     // account for even numbers now, so that we can do i+=2 in loop below
+            return true;
+        } else if (n % 2 == 0) { // account for even numbers now, so that we can do i+=2 in loop below
+            return false;
         }
         int sqrt = (int) Math.sqrt(n);
-        for (int i = 2; i <= sqrt; i++) {
+        for (int i = 3; i <= sqrt; i += 2) { // skips even numbers for faster results
             if (n % i == 0) {
                 return false;
             }
diff --git a/Java/Advanced/Prime Checker/Solution.java b/Java/Advanced/Prime Checker/Solution.java
index 48f712b..5679604 100644
--- a/Java/Advanced/Prime Checker/Solution.java	
+++ b/Java/Advanced/Prime Checker/Solution.java	
@@ -24,15 +24,19 @@ class Prime {
     boolean isPrime(int n) {
         if (n < 2) {
             return false;
+        } else if (n == 2) {     // account for even numbers now, so that we can do i+=2 in loop below
+            return true;
+        } else if (n % 2 == 0) { // account for even numbers now, so that we can do i+=2 in loop below
+            return false;
         }
         int sqrt = (int) Math.sqrt(n);
-        for (int i = 2; i <= sqrt; i++) {
+        for (int i = 3; i <= sqrt; i += 2) { // skips even numbers for faster results
             if (n % i == 0) {
                 return false;
             }
         }
         return true;
-    }   
+    }
 }
 
 public class Solution {
diff --git a/Java/Introduction/Java Date and Time/Solution.java b/Java/Introduction/Java Date and Time/Solution.java
index 360c29a..2c83c25 100644
--- a/Java/Introduction/Java Date and Time/Solution.java	
+++ b/Java/Introduction/Java Date and Time/Solution.java	
@@ -6,16 +6,20 @@ import java.util.Scanner;
 import java.time.LocalDate;
 
 public class Solution {
+    public static String getDay(String day, String month, String year) {
+        int d = Integer.valueOf(day);
+        int m = Integer.valueOf(month);
+        int y = Integer.valueOf(year);
+        LocalDate date = LocalDate.of(y, m, d);
+        return date.getDayOfWeek().toString();
+    }
+
     public static void main(String[] args) {
-        /* Read input */
-        Scanner scan = new Scanner(System.in);
-        int month = scan.nextInt();
-        int day   = scan.nextInt();
-        int year  = scan.nextInt();
-        scan.close();
+        Scanner in = new Scanner(System.in);
+        String month = in.next();
+        String day   = in.next();
+        String year  = in.next();
         
-        /* Create LocalDate instance */
-        LocalDate date = LocalDate.of(year, month, day);
-        System.out.println(date.getDayOfWeek());
+        System.out.println(getDay(day, month, year));
     }
 }