Day 1 (part 2 linear runtime)

day01/part2.py

@@ -1,14 +1,33 @@
 import math
-from itertools import combinations
 
-items = [int(x.strip()) for x in open("input.txt")]
+items = (int(x.strip()) for x in open("input.txt"))
+values = {(2020, 3)}
+tree = dict()
 
-COUNT = 3
 
-for vals in combinations(items, COUNT):
-    if sum(vals) == 2020:
-        print("Found [{}]: Sum={}, Product={}".format(
-            ", ".join(map(str, vals)),
-            sum(vals),
-            math.prod(vals)
-        ))
+def tree_up(idx, step=1, offset = 0):
+    print(idx, step)
+    next = tree[(idx, step)] if (idx, step) in tree else None
+    appendix = [] if next is None else tree_up(next+idx, step=step+1, offset=idx)
+    return [idx - offset] + appendix
+
+
+for item in items:
+    tmp_values = set(values)
+    for elem in values:
+        new = (elem[0] - item, elem[1]-1)
+        if new == (0, 0):
+            vals = tree_up(item)
+            print("Found [{}]: Sum={}, Product={}".format(
+                ", ".join(map(str, vals)),
+                sum(vals),
+                math.prod(vals)
+            ))
+            exit(0)
+        elif new[0] < 0 or new[1] < 0:
+            # target value too low or to many sums
+            continue
+        else:
+            tree[new] = item
+            tmp_values.add(new)
+    values = tmp_values