Day 15, switch to vec for more performance

src/day15.rs

@@ -1,9 +1,8 @@
 use aoc_runner_derive::{aoc, aoc_generator};
-use std::collections::BTreeMap;
-use std::{collections::VecDeque, io::Write};
+use std::collections::HashMap;
 type Data = Vec<usize>;
 
-#[derive(Debug)]
+#[derive(Debug,Clone)]
 enum Seen {
     Never,
     Once(usize),
@@ -45,11 +44,11 @@ pub fn input_generator(input: &str) -> Data {
 }
 
 fn get_number(start: &[usize], num_turns: usize) -> usize {
-    let mut cache: BTreeMap<usize, Seen> = BTreeMap::new();
+    let mut cache = vec![Seen::Never;num_turns];
     let mut nums = start.to_vec();
     let mut num=std::usize::MAX;
     for i in 1..num_turns {
-        num = cache.entry(nums[i - 1]).or_default().add(i);
+        num = cache[nums[i - 1]].add(i);
         if i >= nums.len() {
             nums.push(num);
         }