Big update: Adding all advent of codes to same repo

AdventOfCode2020/day15/day15.java

@@ -0,0 +1,40 @@
+import java.util.ArrayList;
+import java.util.HashMap;
+
+class day15 {
+    
+    public static void main(String[] args) {
+        
+        int[] startingNumbers = {17,1,3,16,19,0};        
+        ArrayList<Integer> spokenNums = new ArrayList<>();
+        HashMap<Integer, Integer> seenLast = new HashMap<>();
+        int lastNumber = 0;
+        
+        for (int i = 0; i < startingNumbers.length; i++) {
+            spokenNums.add(startingNumbers[i]);
+            lastNumber = startingNumbers[i];
+            if (i + 1 != startingNumbers.length) {
+                seenLast.put(startingNumbers[i], i + 1);
+            }
+        }
+
+        while (spokenNums.size() < 30000000) {
+
+            if (!seenLast.containsKey(lastNumber)) {
+                seenLast.put(lastNumber, spokenNums.size());
+                spokenNums.add(0);
+                lastNumber = 0;
+            } else {
+                int diff = spokenNums.size() - seenLast.get(lastNumber);
+                seenLast.put(lastNumber, spokenNums.size());
+                lastNumber = diff;
+                spokenNums.add(lastNumber);
+            }
+
+        }
+        
+        System.out.println("Number: " + spokenNums.get(spokenNums.size() - 1));
+
+    }
+    
+}