Solution for Advent of Code 2015 - Day 1: Not Quite Lisp

Link to the puzzle text

Part 1

AoC 2015 starts simple: use a string to move up and down floors in building and the task is determining the final floor after all instructions. For this we wrote a function to count the number of floor increases/decreases, substract them to get the final floor and return that as the answer.

def part_1(instructions):
    return instructions.count("(") - instructions.count(")")

Testing with the supplied testcases also worked and the answer for the input was also accepted.

Part 2

For the next part we need to figure out which instruction causes our position to go negative. We track the current position by either adding or subtracting 1 to the position and return the current position once we are negative. Maybe there is a sublinear algortihm for this?

def part_2(instructions):
    pos = 0
    for char_pos, char in enumerate(instructions, start=1):
        if char == "(":
            pos += 1
        if char == ")":
            pos -= 1
        if pos == -1:
            return char_pos

    return None

This was again tested against the supplied inputs and actual input.

Link to my solutions