cs: add “1657. Determine if Two Strings Are Close”

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

cs/determine-if-two-strings-are-close.cs

@@ -0,0 +1,33 @@
+public class Solution {
+    private static List<int> Freqs(string word) {
+        var f = new List<int>(Enumerable.Repeat(0, 26));
+
+        foreach (var c in word) {
+            ++f[c - 'a'];
+        }
+
+        return f;
+    }
+
+    private static int Mask(List<int> f) {
+        var m = 0;
+
+        foreach (var c in f) {
+            m = (m << 1) | (c > 0 ? 1 : 0);
+        }
+
+        return m;
+    }
+
+    public bool CloseStrings(string word1, string word2) {
+        var f1 = Freqs(word1);
+        var m1 = Mask(f1);
+        f1.Sort();
+
+        var f2 = Freqs(word2);
+        var m2 = Mask(f2);
+        f2.Sort();
+
+        return m1 == m2 && f1.SequenceEqual(f2);
+    }
+}