cs: add “446. Arithmetic Slices II - Subsequence”

Signed-off-by: Matej Focko <mfocko@redhat.com>

cs/arithmetic-slices-ii-subsequence.cs

@@ -0,0 +1,35 @@
+public class Solution {
+    private static TValue GetOrDefault<TKey, TValue>(Dictionary<TKey, TValue> map, TKey key, TValue defaultValue) {
+        if (map.TryGetValue(key, out var value)) {
+            return value;
+        }
+        return defaultValue;
+    }
+
+    public int NumberOfArithmeticSlices(int[] nums) {
+        var n = nums.Length;
+        var total = 0;
+
+        var dp = new Dictionary<long, int>[n];
+        for (var i = 0; i < n; ++i) {
+            dp[i] = new Dictionary<long, int>();
+        }
+
+        for (var i = 1; i < n; ++i) {
+            for (var j = 0; j < i; ++j) {
+                var diff = (long) nums[i] - nums[j];
+
+                if (diff < int.MinValue || diff > int.MaxValue) {
+                    continue;
+                }
+
+                int count = GetOrDefault(dp[j], diff, 0);
+
+                total += count;
+                dp[i][diff] = 1 + count + GetOrDefault(dp[i], diff, 0);
+            }
+        }
+
+        return total;
+    }
+}