add nim version.

snail.nim

@@ -0,0 +1,62 @@
+
+proc snail(xss: seq[seq[int]]): seq[int] =
+    let h = len(xss)
+    if h == 0:
+        return @[]
+    let w = len(xss[0])
+    if h == 0:
+        return @[]
+    
+    let max = if h > w:
+        h
+    else:
+        w
+
+    var v: seq[int] = @[]
+
+    for b in 0..(max div 2):
+        let ex = w - b - 1;
+        let ey = h - b - 1;
+
+        for c in b..ex:
+            v.add(xss[b][c])
+
+        for r in b+1..ey:
+            v.add(xss[r][ex])
+
+        if ex == b or ey == b:
+            return v
+
+        for o in b..<ex:
+            v.add(xss[ey][ex-1-o])
+
+        for o in b+1..<ey:
+            v.add(xss[ey-o][b])
+
+    return v
+
+var initial = @[@[1,2,3],@[4,5,6],@[7,8,9]]
+var expected = @[1,2,3,6,9,8,7,4,5]
+
+assert expected == snail(initial)
+echo expected == snail(initial)
+ 
+initial = @[@[1, 2, 3, 4], @[5, 6, 7, 8], @[9, 10, 11, 12]]
+expected = @[1, 2, 3, 4, 8, 12, 11, 10, 9, 5, 6, 7]
+
+assert expected == snail(initial)
+echo expected == snail(initial)
+
+initial = @[
+    @[1, 2, 3],
+    @[4, 5, 6],
+    @[7, 8, 9],
+    @[10, 11, 12],
+    @[13, 14, 15]
+]
+expected = @[1, 2, 3, 6, 9, 12, 15, 14, 13, 10, 7, 4, 5, 8, 11]
+
+assert expected == snail(initial)
+echo expected == snail(initial)
+
+echo "all tests passed"