Alternative solution to day 15 (rust)

15.input

@@ -1,4 +1 @@
 0,14,6,20,1,4
-0,3,6
-3,1,2
-1,3,2

15.rs

@@ -0,0 +1,41 @@
+use std::collections::HashMap;
+use std::fs;
+
+fn find_nth(numbers: &[usize], end: usize) -> usize {
+    let numbers: Vec<usize> = numbers.iter().map(|i| *i).collect();
+
+    let mut last_spoken: HashMap<usize, usize> = HashMap::new();
+
+    let mut last: usize = *numbers.last().unwrap();
+
+    let mut numbers: Vec<usize> = numbers[0..numbers.len() - 1].iter().map(|i| *i).collect();
+
+    for (i, num) in numbers.iter().enumerate() {
+        last_spoken.insert(*num, i);
+    }
+
+    for i in numbers.len()..end {
+        let next = match last_spoken.get(&(last)) {
+            Some(index) => i - index,
+            None => 0,
+        };
+        numbers.push(last);
+        last_spoken.insert(last, i);
+        last = next;
+    }
+
+    *numbers.last().unwrap()
+}
+
+fn main() {
+    let num_str = fs::read_to_string("15.input").unwrap();
+    let num_str = num_str.trim();
+
+    let nums: Vec<usize> = num_str
+        .split(",")
+        .map(|s| s.parse::<usize>().unwrap())
+        .collect();
+
+    println!("Answer 1: {}", find_nth(&nums, 2020));
+    println!("Answer 2: {}", find_nth(&nums, 30000000));
+}