advent-of-code-2020/day01/src/main.rs
Tyler Hallada 447940ad7e day01: use HashSet instead for O(n) part1 solution
Also slightly better part2 big O complexity
2020-12-01 23:19:30 -05:00

67 lines
1.7 KiB
Rust

use anyhow::{anyhow, Result};
use std::collections::HashSet;
use std::fs::File;
use std::io::prelude::*;
use std::io::BufReader;
const INPUT: &str = "input/input.txt";
fn solve_part1(input_path: &str) -> Result<i32> {
let file = File::open(input_path)?;
let reader = BufReader::new(file);
let mut prev_numbers = HashSet::new();
for line in reader.lines() {
let number: i32 = line?.parse()?;
let other_half = 2020 - number;
if prev_numbers.contains(&other_half) {
return Ok(number * other_half);
}
prev_numbers.insert(number);
}
Err(anyhow!("Found no pair of numbers that sums to 2020"))
}
fn solve_part2(input_path: &str) -> Result<i32> {
let file = File::open(input_path)?;
let reader = BufReader::new(file);
let mut prev_numbers = HashSet::new();
for line in reader.lines() {
let number: i32 = line?.parse()?;
for prev_number in prev_numbers.iter() {
let other_third = 2020 - prev_number - number;
if prev_numbers.contains(&other_third) {
return Ok(number * prev_number * other_third);
}
}
prev_numbers.insert(number);
}
Err(anyhow!("Found no three numbers that sum to 2020"))
}
fn main() {
println!("Part 1: {}", solve_part1(INPUT).unwrap());
println!("Part 2: {}", solve_part2(INPUT).unwrap());
}
#[cfg(test)]
mod tests {
use super::*;
const TEST_INPUT: &str = "input/test.txt";
#[test]
fn solves_part1() {
assert_eq!(solve_part1(TEST_INPUT).unwrap(), 514579);
}
#[test]
fn solves_part2() {
assert_eq!(solve_part2(TEST_INPUT).unwrap(), 241861950);
}
}