cs: add «3105. Longest Strictly Increasing or Strictly Decreasing Subarray»

URL:	https://leetcode.com/problems/longest-strictly-increasing-or-strictly-decreasing-subarray/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/longest-strictly-increasing-or-strictly-decreasing-subarray.cs

@@ -0,0 +1,20 @@
+public class Solution {
+    private record MonotonicAcc(int Increasing, int Decreasing, int Max) {
+        public MonotonicAcc() : this(1, 1, 1) { }
+
+        public MonotonicAcc Increase() => new(Increasing + 1, 1, Math.Max(Max, Increasing + 1));
+        public MonotonicAcc Decrease() => new(1, Decreasing + 1, Math.Max(Max, Decreasing + 1));
+        public MonotonicAcc Reset() => new(1, 1, Max);
+    }
+
+    public int LongestMonotonicSubarray(int[] nums) =>
+        nums.Zip(nums.Skip(1)).Aggregate(
+            new MonotonicAcc(),
+            (acc, pair) => pair switch {
+                _ when pair.First < pair.Second => acc.Increase(),
+                _ when pair.First > pair.Second => acc.Decrease(),
+                _ => acc.Reset(),
+            }
+        ).Max;
+
+}