python solutions through day17

2024/day10/day10.py

@@ -0,0 +1,99 @@
+def part1(input: str) -> int:
+    grid: list[list[int, int]] = []
+    for line in input.splitlines():
+        grid.append([int(x) for x in list(line)])
+    # alternative pythonic way...
+    # grid = [[int(char) for char in line] for line in input.splitlines()]
+
+    ans: int = 0
+    for r in range(len(grid)):
+        for c in range(len(grid[0])):
+            if grid[r][c] == 0:
+                ans += len(dfs_backtrack_unique_end_coords(grid, r, c, {(r, c)}))
+
+    return ans
+
+
+# I misread the instructions slightly and went for a "unique paths" algo at first.
+# It would be simpler to pass the "reachable 9s coords" set in as an arg and update
+#   it in the termination case, then take the length of that set in the part1() function.
+#   This would remove the need to combine the sets which is potentially expensive (and ugly)
+def dfs_backtrack_unique_end_coords(
+    grid: list[list[int, int]], row: int, col: int, visited: set[tuple[int, int]]
+) -> set[tuple[int, int]]:
+    if grid[row][col] == 9:
+        return {(row, col)}
+
+    all_coords: set[tuple[int, int]] = set()
+    for diff in [(-1, 0), (1, 0), (0, -1), (0, 1)]:
+        nextRow = row + diff[0]
+        nextCol = col + diff[1]
+
+        if not (0 <= nextRow < len(grid) and 0 <= nextCol < len(grid[0])):
+            continue
+        if (nextRow, nextCol) in visited:
+            continue
+        if grid[nextRow][nextCol] == grid[row][col] + 1:
+            visited.add((nextRow, nextCol))
+            new_coords = dfs_backtrack_unique_end_coords(
+                grid, nextRow, nextCol, visited
+            )
+            # combines the two sets, see comment above function def
+            all_coords.update(new_coords)
+            visited.remove((nextRow, nextCol))
+
+    return all_coords
+
+
+def part2(input: str) -> int:
+    grid = [[int(char) for char in line] for line in input.splitlines()]
+
+    ans: int = 0
+    for r in range(len(grid)):
+        for c in range(len(grid[0])):
+            if grid[r][c] == 0:
+                ans += dfs_backtrack_unique_paths(grid, r, c, {(r, c)})
+
+    return ans
+
+
+def dfs_backtrack_unique_paths(
+    grid: list[list[int, int]], row: int, col: int, visited: set[tuple[int, int]]
+) -> int:
+    if grid[row][col] == 9:
+        # unique path found
+        return 1
+
+    total: int = 0
+    for diff in [(-1, 0), (1, 0), (0, -1), (0, 1)]:
+        nextRow = row + diff[0]
+        nextCol = col + diff[1]
+
+        if not (0 <= nextRow < len(grid) and 0 <= nextCol < len(grid[0])):
+            continue
+        if (nextRow, nextCol) in visited:
+            continue
+        if grid[nextRow][nextCol] == grid[row][col] + 1:
+            visited.add((nextRow, nextCol))
+            total += dfs_backtrack_unique_paths(grid, nextRow, nextCol, visited)
+            visited.remove((nextRow, nextCol))
+
+    return total
+
+
+example = """89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732"""
+
+input = open("input.txt").read().strip()
+
+print(f"part1 example: {part1(example)} want 36")
+print(f"part1: {part1(input)} want 782")
+
+print(f"part2 example: {part2(example)} want 81")
+print(f"part2 example: {part2(input)} want 1694")