day3: add python solution

README.md

@@ -8,3 +8,4 @@ https://adventofcode.com/2020/
 |---|--------|---------|------|
 | 1 |  `**`  |  `**`   |      |
 | 2 |  `**`  |         | `**` |
+| 3 |  `**`  |         |      |

day3/day3.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python
+
+def count_trees(right, down, lines):
+    n = 0
+    for i, line in enumerate(lines[::down]):
+        c = line[(i*right) % len(line)]
+        if c == '#':
+            n += 1
+    return n
+
+if __name__ == '__main__':
+    with open('input.txt', 'r') as f:
+        lines = [line.strip() for line in f.readlines()]
+
+    print(count_trees(3, 1, lines))
+
+    hits_multiplied = 1
+    for slope in ((1,1), (3,1), (5,1), (7,1), (1,2)):
+        hits_multiplied *= count_trees(*slope, lines)
+    print(hits_multiplied)