cs: add «567. Permutation in String»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/permutation-in-string.cs

@@ -0,0 +1,36 @@
+public class Solution {
+    private int[] GetFreqs(string s, int begin, int end) {
+        var freqs = new int[26];
+
+        for (int i = begin; i < end; ++i) {
+            ++freqs[s[i] - 'a'];
+        }
+
+        return freqs;
+    }
+    private int[] GetFreqs(string s) => GetFreqs(s, 0, s.Length);
+
+    public bool CheckInclusion(string s1, string s2) {
+        if (s2.Length < s1.Length) {
+            return false;
+        }
+
+        var expected = GetFreqs(s1);
+
+        var running = GetFreqs(s2, 0, s1.Length);
+        if (Enumerable.SequenceEqual(running, expected)) {
+            return true;
+        }
+
+        for (int i = s1.Length; i < s2.Length; ++i) {
+            --running[s2[i - s1.Length] - 'a'];
+            ++running[s2[i] - 'a'];
+
+            if (Enumerable.SequenceEqual(running, expected)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}