add rust version.

snail.rs

@@ -0,0 +1,56 @@
+fn snail<T>(xss: &[Vec<T>]) -> Vec<T>
+where
+    T: Copy,
+{
+    let h: usize = xss.len();
+    let w: usize = xss.get(0).map(Vec::len).unwrap_or(0);
+
+    if w == 0 || h == 0 {
+        return Vec::new();
+    }
+
+    (0..=(h.max(w) / 2)).fold(Vec::with_capacity(w * h), |mut v, b| {
+        let ex = w - b - 1;
+        let ey = h - b - 1;
+
+        let it = (b..=ex)
+            .map(|c| xss[b][c])
+            .chain(((b + 1)..=ey).map(|r| xss[r][ex]));
+
+        if ex == b || ey == b {
+            v.extend(it);
+            v
+        } else {
+            v.extend(
+                it.chain((b..ex).rev().map(|c| xss[ey][c]))
+                    .chain(((b + 1)..ey).rev().map(|r| xss[r][b])),
+            );
+
+            v
+        }
+    })
+}
+
+fn main() {
+    let initial = &[vec![1, 2, 3], vec![4, 5, 6], vec![7, 8, 9]];
+    let expected = vec![1, 2, 3, 6, 9, 8, 7, 4, 5];
+    assert_eq!(expected, snail(initial));
+    println!("{}", expected == snail(initial));
+
+    let initial = &[vec![1, 2, 3, 4], vec![5, 6, 7, 8], vec![9, 10, 11, 12]];
+    let expected = vec![1, 2, 3, 4, 8, 12, 11, 10, 9, 5, 6, 7];
+    assert_eq!(expected, snail(initial));
+    println!("{}", expected == snail(initial));
+
+    let initial = &[
+        vec![1, 2, 3],
+        vec![4, 5, 6],
+        vec![7, 8, 9],
+        vec![10, 11, 12],
+        vec![13, 14, 15],
+    ];
+    let expected = vec![1, 2, 3, 6, 9, 12, 15, 14, 13, 10, 7, 4, 5, 8, 11];
+    assert_eq!(expected, snail(initial));
+    println!("{}", expected == snail(initial));
+    println!("all tests passed");
+}