thallada/advent-of-code-2020
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day01: use HashSet instead for O(n) part1 solution

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Also slightly better part2 big O complexity
This commit is contained in:
Tyler Hallada 2020-12-01 23:19:30 -05:00
parent 9b5fb1cdc4
commit 447940ad7e
1 changed files with 16 additions and 17 deletions
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33
day01/src/main.rs
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@ -1,44 +1,43 @@
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<u32> {
fn solve_part1(input_path: &str) -> Result<i32> {
let file = File::open(input_path)?;
let reader = BufReader::new(file);
let mut prev_lines = Vec::new();
let mut prev_numbers = HashSet::new();
for line in reader.lines() {
let number: u32 = line?.parse()?;
for prev_number in prev_lines.iter() {
if number + prev_number == 2020 {
return Ok(number * prev_number);
}
let number: i32 = line?.parse()?;
let other_half = 2020 - number;
if prev_numbers.contains(&other_half) {
return Ok(number * other_half);
}
prev_lines.push(number);
prev_numbers.insert(number);
}
Err(anyhow!("Found no pair of numbers that sums to 2020"))
}
fn solve_part2(input_path: &str) -> Result<u32> {
fn solve_part2(input_path: &str) -> Result<i32> {
let file = File::open(input_path)?;
let reader = BufReader::new(file);
let mut prev_lines = Vec::new();
let mut prev_numbers = HashSet::new();
for line in reader.lines() {
let number: u32 = line?.parse()?;
for prev_number in prev_lines.iter() {
for third_number in prev_lines.iter() {
if number + prev_number + third_number == 2020 {
return Ok(number * prev_number * third_number);
}
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_lines.push(number);
prev_numbers.insert(number);
}
Err(anyhow!("Found no three numbers that sum to 2020"))