cs: add «386. Lexicographical Numbers»

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

cs/lexicographical-numbers.cs

@@ -0,0 +1,23 @@
+public class Solution {
+    public IList<int> LexicalOrder(int n) {
+        var numbers = new List<int>();
+
+        var current = 1;
+        for (int i = 0; i < n; ++i) {
+            numbers.Add(current);
+
+            if (current * 10 <= n) {
+                current *= 10;
+                continue;
+            }
+
+            // backtrack and try next
+            while (current >= n || current % 10 == 9) {
+                current /= 10;
+            }
+            ++current;
+        }
+
+        return numbers;
+    }
+}