cs: add «2179. Count Good Triplets in an Array»

URL:	https://leetcode.com/problems/count-good-triplets-in-an-array/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/count-good-triplets-in-an-array.cs

@@ -0,0 +1,56 @@
+public class Solution {
+    private class FenwickTree {
+        private int[] tree;
+
+        public FenwickTree(int size) {
+            tree = new int[size + 1];
+        }
+
+        public int this[int i] {
+            get {
+                var res = 0;
+
+                for (++i; i > 0; i -= i & -i) {
+                    res += tree[i];
+
+                }
+
+                return res;
+            }
+            set {
+                for (++i; i < tree.Length; i += i & -i) {
+                    tree[i] += value;
+                }
+            }
+        }
+    }
+
+    public long GoodTriplets(int[] nums1, int[] nums2) {
+        var n = nums1.Length;
+
+        var pos2 = new int[n];
+        for (var i = 0; i < n; ++i) {
+            pos2[nums2[i]] = i;
+        }
+
+        var reversedMapping = new int[n];
+        for (var i = 0; i < n; ++i) {
+            reversedMapping[pos2[nums1[i]]] = i;
+        }
+
+        long count = 0;
+
+        var t = new FenwickTree(n);
+        for (var value = 0; value < n; ++value) {
+            var pos = reversedMapping[value];
+
+            var left = t[pos];
+            t[pos] = 1;
+
+            var right = (n - 1 - pos) - (value - left);
+            count += (long)left * right;
+        }
+
+        return count;
+    }
+}