cs: add «345. Reverse Vowels of a String»

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

cs/reverse-vowels-of-a-string.cs

@@ -0,0 +1,26 @@
+public class Solution {
+    private bool IsVowel(char c) {
+        return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u';
+    }
+
+    public string ReverseVowels(string s) {
+        var reversed = new StringBuilder(s);
+
+        s = s.ToLower();
+        for (int l = 0, r = reversed.Length - 1; l < r; ++l, --r) {
+            while (l < reversed.Length && !IsVowel(s[l])) {
+                ++l;
+            }
+
+            while (r > 0 && !IsVowel(s[r])) {
+                --r;
+            }
+
+            if (l < r) {
+                (reversed[l], reversed[r]) = (reversed[r], reversed[l]);
+            }
+        }
+
+        return reversed.ToString();
+    }
+}