Add part one of day 2

data/examples/02.txt

@@ -0,0 +1,6 @@
+7 6 4 2 1
+1 2 7 8 9
+9 7 6 2 1
+1 3 2 4 5
+8 6 4 4 1
+1 3 6 7 9

src/bin/02.rs

@@ -1,7 +1,23 @@
 advent_of_code::solution!(2);
 
 pub fn part_one(input: &str) -> Option<u32> {
-    None
+    let mut num_safe: u32 = 0;
+    let lines = input.lines();
+    for line in lines {
+        let levels: Vec<u32> = line
+            .split_whitespace()
+            .filter_map(|i| i.parse::<u32>().ok())
+            .collect();
+
+        if levels
+            .windows(2)
+            .all(|w| (w[0] > w[1] && w[0] - w[1] <= 3) || (w[1] > w[0] && w[1] - w[0] <= 3))
+            && (levels.windows(2).all(|w| w[0] < w[1]) || levels.windows(2).all(|w| w[0] > w[1]))
+        {
+            num_safe += 1;
+        }
+    }
+    Some(num_safe)
 }
 
 pub fn part_two(input: &str) -> Option<u32> {
@@ -15,7 +31,7 @@ mod tests {
     #[test]
     fn test_part_one() {
         let result = part_one(&advent_of_code::template::read_file("examples", DAY));
-        assert_eq!(result, None);
+        assert_eq!(result, Some(2));
     }
 
     #[test]