cs: add «1790. Check if One String Swap Can Make Strings Equal»

URL:	https://leetcode.com/problems/check-if-one-string-swap-can-make-strings-equal/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/check-if-one-string-swap-can-make-strings-equal.cs

@@ -0,0 +1,33 @@
+using Pairing = (char, char);
+
+public class Solution {
+    private static readonly Pairing UNSET = ('\0', '\0');
+
+    private record Diff(int Count, Pairing First, Pairing Second) {
+        public Diff() : this(0, UNSET, UNSET) { }
+
+        public Diff Update(char l, char r) {
+            if (l == r) {
+                return this;
+            }
+
+            return new Diff(
+                Count + 1,
+                First == UNSET ? (l, r) : First,
+                First != UNSET && Second == UNSET ? (l, r) : Second
+            );
+        }
+
+        public bool CanSwap => Count == 0 || (
+            Count == 2
+            && First.Item1 == Second.Item2
+            && First.Item2 == Second.Item1
+        );
+    }
+
+    public bool AreAlmostEqual(string s1, string s2) =>
+        s1.Zip(s2).Aggregate(new Diff(), (acc, chars) => {
+            var (l, r) = chars;
+            return acc.Update(l, r);
+        }).CanSwap;
+}