2020 day15/python: add solution

2020/README.md

@@ -19,5 +19,6 @@ https://adventofcode.com/2020/
 |11 |  `**`  |         |      |
 |13 |        |  `**`   |      |
 |14 |        |  `**`   |      |
+|15 |  `**`  |         |      |
 
 `test.sh` can be used to run all solutions and automatically compares them to (my) puzzle inputs and the expected outputs.

2020/data/day15.expected

@@ -0,0 +1,2 @@
+496
+883

2020/data/day15.input

@@ -0,0 +1 @@
+2,0,1,7,4,14,18

2020/day15/day15.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+
+import sys
+
+def run_game(init, iterations):
+    last_seen = {}
+    previous = None
+
+    for i, n in enumerate(init):
+        if previous is not None:
+            last_seen[previous] = i-1
+        previous = n
+
+    for i in range(i+1, iterations):
+        if previous in last_seen:
+            x = i - last_seen[previous] - 1
+        else:
+            x = 0
+        last_seen[previous] = i-1
+        previous = x
+
+    return previous
+
+if __name__ == '__main__':
+    init = [int(n) for n in sys.stdin.read().split(',')]
+    print(run_game(init, 2020))
+    print(run_game(init, 30000000))